Chromosome 1 is the largest human chromosome, carrying roughly 2,000 protein-coding genes that influence an extraordinary range of biological functions, from brain size and cholesterol levels to blood type and skin integrity. Because it contains about eight percent of the entire human genome, chromosome 1 touches nearly every organ system in the body. No single summary can capture everything it does, but the genes housed on this chromosome play documented roles in heart disease, cancer, neurodegeneration, immune defense, vision, and even the evolutionary expansion of the human brain.
Brain Size and Development
Two gene families on chromosome 1 stand out for their connection to brain growth. The first is ASPM, a gene whose protein helps organize the cellular machinery that divides neural stem cells during fetal development. When ASPM is knocked out by certain mutations, the result is primary microcephaly, a condition marked by roughly a 70 percent reduction in brain size.1PubMed Central. Evolution of the human ASPM gene, a major determinant of brain size The gene itself has been under strong evolutionary selection in primates, suggesting it was one of the drivers behind the increase in brain volume along the lineage leading to modern humans.
The second family, called NOTCH2NL, is even more specifically human. These genes are paralogs of a signaling receptor called NOTCH2, and they appear to exist only in humans and our closest extinct relatives. Functional studies show that NOTCH2NL genes promote the self-renewal of cortical progenitor cells, the stem-like cells that eventually become neurons in the outer brain. By keeping those progenitors dividing longer before they mature, NOTCH2NL genes increase total neuronal output, which contributes to the larger, more folded neocortex that distinguishes human brains.2Cell. Human-Specific NOTCH2NL Genes Expand Cortical Progenitors and The Human Neocortex Evolutionary analyses suggest these genes arose through duplication events that are still ongoing, with natural variants trending toward reduced protein levels over time.3PubMed Central. Evolution of Human Brain Size-Associated NOTCH2NL Genes Proceeds toward Reduced Protein Levels The region where NOTCH2NL sits is structurally unstable, and deletions or duplications there have been linked to macrocephaly and microcephaly, as well as autism and schizophrenia.
Cholesterol and Heart Disease
One of the most medically consequential genes on chromosome 1 is PCSK9, which encodes a protein that regulates how many cholesterol receptors sit on the surface of liver cells. When PCSK9 levels are high, the liver pulls fewer cholesterol particles out of the bloodstream, and LDL cholesterol rises. Genetic variants in PCSK9 can push cholesterol levels in either direction: gain-of-function mutations raise LDL and increase the risk of atherosclerosis, while loss-of-function variants lower LDL and appear to be protective against heart disease.4PubMed. The influence of PCSK9 polymorphisms on serum low-density lipoprotein cholesterol and risk of atherosclerosis
This discovery has already reshaped cardiovascular medicine. A class of injectable drugs called PCSK9 inhibitors, which block the protein and let the liver clear more cholesterol, has been in clinical use for several years. More recently, researchers have developed VERVE-101, a CRISPR-based gene-editing therapy designed to permanently inactivate the PCSK9 gene in liver cells, potentially offering a one-time treatment that keeps LDL low for life.5PubMed. VERVE-101, a CRISPR base-editing therapy designed to permanently inactivate hepatic PCSK9 and reduce LDL-cholesterol PCSK9 is one of the clearest examples of how a single gene on chromosome 1 has gone from basic science curiosity to an active drug target in under two decades.
Blood Type, Skin Barrier, and Immunity
Your Rh blood type, the “positive” or “negative” that follows your ABO group, traces back to chromosome 1. Two adjacent genes called RHD and RHCE sit on the short arm at position 1p34-1p36 and encode the Rh proteins on the surface of red blood cells.6Blood. RHD gene deletion occurred in the Rhesus box People who are Rh-negative typically carry a deletion of the entire RHD gene. Rh incompatibility between a pregnant person and their fetus can trigger hemolytic disease of the newborn if not managed, making this one of the most practically significant gene deletions on any chromosome.
Chromosome 1 also houses the filaggrin gene (FLG), which encodes a protein critical for forming the outermost layer of skin. Filaggrin helps flatten dying skin cells into the tough, water-resistant shield that keeps moisture in and allergens out. Loss-of-function mutations in FLG lead to a “leaky” skin barrier, causing excessive water loss through the skin, the dry and scaly texture characteristic of eczema, and easier penetration of environmental allergens.7PubMed Central. The allergy gene: how a mutation in a skin protein revealed a link between eczema and asthma Studies in infants as young as three months have shown that those carrying FLG mutations already have measurably higher water loss through the skin, even before visible eczema appears, suggesting the barrier defect comes first and the inflammation follows.8British Journal of Dermatology. Filaggrin loss‐of‐function mutations are associated with early‐onset eczema, eczema severity and transepidermal water loss at 3 months of age Filaggrin mutations are now recognized as one of the strongest known genetic risk factors for developing both eczema and, later, allergic asthma.
The immune system gets further support from chromosome 1 through several complement genes. The complement system is a cascade of proteins that help antibodies and white blood cells clear pathogens and dead cells. More than 45 genes across the genome encode complement proteins, but chromosome 1 contributes to key clusters. When early components of this cascade are missing due to genetic deficiency, the body struggles to clear immune complexes and cellular debris, raising the risk of autoimmune diseases.9Oxford Academic (Protein & Cell). Complement genetics, deficiencies, and disease associations
1p36 Deletion Syndrome
The tip of chromosome 1’s short arm is the site of one of the most common chromosomal deletion disorders. When a piece of the 1p36 region is missing, the result is 1p36 deletion syndrome, estimated to occur in roughly 1 in 5,000 to 1 in 10,000 births.10PubMed Central. 1p36 deletion syndrome: Review and mapping with further characterization of the phenotype, a new cohort of 86 patients The syndrome produces a recognizable pattern of features that can vary depending on how large the deletion is and exactly where it falls.
In a detailed study of 60 patients, virtually all had developmental delay with poor or absent speech, and 95 percent had low muscle tone. About 71 percent had heart defects, including a distinctive form of cardiomyopathy, while 88 percent showed structural differences in the brain and 44 percent had seizures. Facial features tend to include straight eyebrows, deep-set eyes, a broad nasal bridge, and a pointed chin.11Pediatrics. Further Delineation of Deletion 1p36 Syndrome in 60 Patients: A Recognizable Phenotype and Common Cause of Developmental Delay and Mental Retardation Some of the most serious complications, including the cardiomyopathy and seizures, can often be managed with medication, and all patients showed continued developmental progress over time. Mapping studies have identified at least four critical regions within the 1p36 band that appear to be responsible for intellectual disability, which helps explain why symptoms can differ so much from one patient to another.10PubMed Central. 1p36 deletion syndrome: Review and mapping with further characterization of the phenotype, a new cohort of 86 patients
Cancer Connections
Chromosome 1 abnormalities show up across several types of cancer, but two patterns are especially well studied: deletions on the short arm (1p) and gains on the long arm (1q).
In neuroblastoma, a childhood cancer of the nervous system, loss of material at 1p36 is one of the most established markers of aggressive disease. In a large study of nearly 900 tumors, about 23 percent had lost genetic material at 1p36, and that loss was strongly associated with amplification of the MYCN oncogene and with high-risk disease features. Patients with 1p36 loss who were otherwise classified as low or intermediate risk still had significantly worse progression-free survival.12PubMed. Chromosome 1p and 11q deletions and outcome in neuroblastoma Genome-wide analyses have confirmed that 1p loss and MYCN amplification are tightly linked in these tumors.13PLoS ONE. Genome-Wide Analysis of Neuroblastomas using High-Density Single Nucleotide Polymorphism Arrays This means that for clinicians staging neuroblastoma, testing for 1p36 deletion helps predict which patients need more aggressive treatment.
On the other arm of the chromosome, extra copies of the 1q region are a growing concern in multiple myeloma, a cancer of plasma cells in the bone marrow. The international staging system for myeloma now recognizes a gain at 1q as a high-risk feature, with increasing evidence that it shortens the time patients stay in remission.14PubMed Central. Overview of 1q abnormalities in multiple myeloma: scientific opinions from Italian experts In patients who underwent stem cell transplant at one major cancer center, those with 1q amplification had roughly double the risk of disease progression compared to those without it.15Blood Cancer Journal. Outcomes of patients with multiple myeloma and 1q gain/amplification receiving autologous hematopoietic stem cell transplant
Chromosome 1 also carries ARID1A, a tumor suppressor gene that helps regulate how cells read their DNA and progress through the cell cycle. ARID1A mutations have been found in a broad spectrum of cancers but are especially common in cancers related to the uterine lining. The gene functions both as a “gatekeeper,” preventing cells from dividing unchecked, and as a “caretaker,” maintaining genomic stability so that other dangerous mutations are less likely to accumulate.16PubMed Central. The emerging roles of ARID1A in tumor suppression
Parkinson’s Disease and Premature Aging
Chromosome 1 hosts two genes with very different but equally striking connections to aging and neurodegeneration. The GBA1 gene encodes an enzyme called glucocerebrosidase, which breaks down certain fatty molecules inside lysosomes, the recycling centers of cells. Complete loss of this enzyme causes Gaucher disease, a rare storage disorder, but even partial loss turns out to be among the most common known genetic risk factors for Parkinson’s disease.17PubMed Central. Glucocerebrosidase and its relevance to Parkinson disease People with GBA-associated parkinsonism tend to develop symptoms at a younger age and experience more cognitive changes than those whose Parkinson’s has no GBA link. Researchers believe the connection involves a protein called alpha-synuclein: when glucocerebrosidase is impaired, alpha-synuclein may accumulate more readily, forming the toxic clumps that are a hallmark of Parkinson’s pathology.18PubMed Central. The link between the GBA gene and parkinsonism
A very different face of aging sits in the LMNA gene, also on chromosome 1. LMNA encodes lamins, structural proteins that form a mesh just inside the nuclear membrane, giving cells their shape and organizing how DNA is accessed. A specific splicing mutation in LMNA produces an abnormal protein called progerin, which causes Hutchinson-Gilford progeria syndrome, a devastating condition in which children age at roughly five to ten times the normal rate. Progerin accumulation leads to misshapen cell nuclei, DNA repair defects, shortened telomeres, and genomic instability, all of which exhaust cells’ ability to divide.19PubMed Central. Hutchinson-Gilford Progeria Syndrome: A premature aging disease caused by LMNA gene mutations Progeria is extremely rare, but studying it has given researchers unexpected insights into normal aging, because healthy elderly people also accumulate low levels of progerin in their cells.
Vision Loss and Stargardt Disease
Chromosome 1 carries ABCA4, a gene active in the light-sensing cells of the retina. ABCA4 encodes a protein that clears toxic byproducts of the visual cycle from photoreceptor cells. When this gene is mutated, those waste products build up and progressively destroy the central part of the retina, causing Stargardt disease, the most common inherited form of juvenile macular degeneration. One well-characterized mutation, a single base change deep in an intron, causes the splicing machinery to skip one or two critical segments of the gene’s instructions. Patients who carry this variant on both copies of ABCA4 tend to develop severe cone-rod dystrophy before the age of 20 and become legally blind by 25.20PubMed. Photoreceptor Progenitor mRNA Analysis Reveals Exon Skipping Resulting from the ABCA4 c.5461-10T→C Mutation in Stargardt Disease Because many ABCA4 mutations are “subtle” by standard genetic testing standards, Stargardt disease is a case where deeper analysis at the RNA level is needed to find the true cause.
How Chromosome 1 Shaped Human Evolution
Beyond its clinical relevance, chromosome 1 offers a window into primate evolution. Repetitive DNA sequences unique to chromosome 1 have been found across the primate family tree, present in apes, Old World monkeys, and New World monkeys, but absent in the more distantly related prosimians such as lemurs.21PubMed. Primate evolution of a human chromosome 1 hypervariable repetitive element Interestingly, the copy number and arrangement of these repeats do not always track neatly with accepted evolutionary relationships, which suggests that repetitive elements can evolve through mechanisms like gene conversion and unequal crossing-over that scramble the usual phylogenetic signals.
The NOTCH2NL gene family discussed earlier offers a more dramatic evolutionary story. These genes appear to have arisen through a series of duplication events specific to the human lineage, some as recently as a few million years ago. Their role in expanding cortical progenitor pools makes them strong candidates for explaining why human brains are disproportionately large relative to body size.2Cell. Human-Specific NOTCH2NL Genes Expand Cortical Progenitors and The Human Neocortex The fact that this region of chromosome 1 is structurally prone to rearrangement means it is both an engine of evolutionary innovation and a source of vulnerability, since the same instability that generated new brain-growth genes also predisposes to pathogenic deletions and duplications.
Prenatal Screening for Chromosome 1 Abnormalities
Detecting problems on chromosome 1 before birth has become increasingly feasible. Traditional prenatal screening can flag large chromosomal imbalances, but newer approaches are pushing the resolution much finer. A cell-based method that isolates fetal cells from a maternal blood sample has demonstrated the ability to detect deletions and duplications as small as 1 to 2 megabases, with results that matched conventional genetic testing in all cases examined.22PubMed Central. Reliable detection of subchromosomal deletions and duplications using cell-based noninvasive prenatal testing For conditions like 1p36 deletion syndrome, where early diagnosis can guide cardiac monitoring and seizure management, the ability to identify the deletion from a simple blood draw rather than an invasive procedure represents a meaningful advance for affected families. As these technologies continue to improve, chromosome 1’s large size and dense gene content make it a particularly important target for prenatal and postnatal genomic analysis.