The most severe form is beta thalassemia major, where the person has two β0 mutations, so no β-globin is produced. Beta thalassemia intermedia occurs when one or both defective copies carries a β+ mutation so some β-globin can still be produced. Beta thalassemia minor (carrier) occurs when only one copy of the HBB gene carries a mutation. Beta thalassemia mutations can also be inherited with sickle cell mutations, resulting in sickle/beta thalassemia disorders of varying severity.