Monarch Disease Ontology id MONDO_0019107 (Rh deficiency syndrome) can be described as follows. The Rh deficiency syndrome, also known as Rh-null syndrome, is a blood disorder where people have red blood cells (RBCs) lacking all Rh antigens. The Rh antigens maintain the integrity of the RBC membrane and therefore, RBCs which lack Rh antigens have an abnormal shape. There are two types of Rh deficiency syndrome: The regulator type is associated with many different changes (mutations) in the RHAG gene. The amorph type is caused by inactive copies of a gene (silent alleles) at the RH locus. As a result, the RBCs do not express any of the Rh antigens. The absence of the Rh complex alters the RBC shape, increases its tendency to break down (osmotic fragility), and shortens its lifespan, resulting in a hemolytic anemia that is usually mild. These patients are at risk of having adverse transfusion reactions because they may produce antibodies against several of the Rh antigens and can only receive blood from people who have the same condition. Rh deficiency syndrome is inherited in an autosomal recessive manner. Management is individualized according to the severity of hemolytic anemia. Also known as: Rh-null syndrome, anemia, hemolytic, Rh-null, regulator type.