The HBB gene carries the instructions your body needs to make beta-globin, a protein that forms one of the core building blocks of hemoglobin. Hemoglobin is the molecule inside red blood cells responsible for picking up oxygen in your lungs and delivering it to every tissue in your body. Mutations in HBB are behind some of the most common inherited blood disorders in the world, including sickle cell disease and beta-thalassemia.
What the HBB Gene Does
HBB stands for “hemoglobin subunit beta.” The gene sits on chromosome 11, at a location designated 11p15.4, and it contains three exons, the segments of DNA that code for the final protein.
The protein it produces, beta-globin, pairs up with another protein called alpha-globin (made by a separate gene on chromosome 16) to form adult hemoglobin, known as hemoglobin A. Each hemoglobin molecule is built from four protein subunits: two beta-globin and two alpha-globin. Every one of those subunits holds an iron-containing molecule called heme, and the iron atom at the center of each heme can latch onto one oxygen molecule. That means a single hemoglobin molecule carries up to four oxygen molecules at a time, which is how red blood cells efficiently shuttle oxygen from your lungs to the rest of your body.
The Fetal-to-Adult Hemoglobin Switch
Before birth, your body relies on a different version of hemoglobin called fetal hemoglobin (HbF), which uses gamma-globin chains instead of beta-globin. Shortly after birth, a molecular switch flips: genes producing gamma-globin gradually quiet down, and the HBB gene ramps up to produce beta-globin. This transition is controlled by a protein called BCL11A, which acts as a silencer of the fetal gamma-globin genes. As BCL11A levels rise in maturing red blood cell precursors, fetal hemoglobin drops and adult hemoglobin takes over.
This switch matters clinically because fetal hemoglobin doesn’t cause sickling. Therapies that reactivate fetal hemoglobin, by targeting BCL11A or related pathways, are now a major strategy for treating sickle cell disease.
Sickle Cell Disease: The Most Well-Known HBB Mutation
Sickle cell disease results from a single, specific change in the HBB gene. One DNA letter, adenine, is swapped for thymine at position 20 of the coding sequence. That tiny change replaces glutamic acid, a charged, water-friendly amino acid, with valine, a neutral, water-repelling one. The swap alters the surface chemistry of the beta-globin protein just enough that, when oxygen levels drop, hemoglobin molecules stick together and form rigid fibers inside red blood cells. The cells distort into the characteristic crescent or “sickle” shape, becoming stiff and prone to clogging small blood vessels.
The result is episodes of severe pain (called vaso-occlusive crises), chronic anemia, organ damage over time, and a shortened lifespan without treatment. A person needs two copies of the sickle mutation (one from each parent) to develop the full disease. Carrying just one copy, known as sickle cell trait, typically causes no symptoms and actually provides some protection against malaria, which helps explain why the mutation is so common in regions where malaria has historically been widespread.
Beta-Thalassemia: Too Little Beta-Globin
While sickle cell disease involves a structurally abnormal beta-globin protein, beta-thalassemia involves making too little of it, or none at all. Over 200 different HBB mutations can cause thalassemia, and they fall into two broad categories. Beta-zero mutations completely shut down beta-globin production from that copy of the gene. Beta-plus mutations, often found in the gene’s promoter region, reduce production but don’t eliminate it entirely. The severity of the disease depends heavily on which combination a person inherits.
Someone with two beta-zero mutations has beta-thalassemia major, the most severe form, and typically requires regular blood transfusions starting in infancy. Two beta-plus mutations, or one of each, can produce a range of severity from moderate (thalassemia intermedia) to mild. Carriers with just one affected copy usually have no symptoms, though blood tests may show slightly smaller red blood cells than average.
Other HBB Variants
Sickle cell and thalassemia get the most attention, but HBB has other clinically relevant variants. Hemoglobin C results from a different amino acid substitution at the same position as the sickle mutation. People with two copies of hemoglobin C (HbCC) tend to have mild chronic anemia, an enlarged spleen, and sometimes gallstones from the gradual breakdown of red blood cells. It’s most prevalent in West Africa and parts of Southeast Asia, again in areas with historic malaria exposure.
Hemoglobin E is another common variant, particularly in Southeast Asian populations. Like hemoglobin C and S, it likely persisted through evolution because carrying one copy offers some defense against malaria. Combinations of these variants, such as inheriting one sickle gene and one hemoglobin C gene (HbSC disease), can produce their own distinct clinical pictures, sometimes nearly as serious as sickle cell disease.
How HBB Disorders Are Inherited
HBB-related conditions follow an autosomal recessive pattern, meaning a child must inherit a mutated copy of the gene from both parents to develop the disease. If both parents are carriers (each with one normal and one mutated copy), each pregnancy carries a 25% chance of producing an affected child, a 50% chance of producing another carrier, and a 25% chance of a child with two normal copies. If one parent is a carrier and the other has the disease, the odds shift to 50/50 between being affected and being a carrier.
Carrier screening is straightforward and widely available. A simple blood test called hemoglobin electrophoresis can identify whether someone carries an abnormal hemoglobin variant. Newborn screening programs in many countries now routinely test for sickle cell disease at birth.
Gene Therapies Targeting HBB
In December 2023, the FDA approved two gene therapies for sickle cell disease in patients 12 and older, marking a major milestone. Casgevy, developed by Vertex Pharmaceuticals, uses CRISPR gene-editing technology to disable the BCL11A gene in a patient’s own blood stem cells. With BCL11A silenced, those cells resume producing fetal hemoglobin, which doesn’t sickle. Lyfgenia, from Bluebird Bio, takes a different approach: it uses a modified virus to insert a functional, anti-sickling version of the beta-globin gene into the patient’s stem cells.
Both therapies require collecting a patient’s stem cells, modifying them in a lab, then infusing them back after chemotherapy clears the existing bone marrow. The process is intensive, involving weeks in the hospital, but the goal is a one-time treatment that provides lasting relief from pain crises and the other complications of sickle cell disease. Clinical trials for similar approaches in beta-thalassemia are also advancing, with Casgevy already approved for transfusion-dependent thalassemia as well.