It's a rare autosomal recessive disorder characterized by hypopigmentation of skin and hair, the presence of large aggregates of pigment in the hair shafts and the accumulation of mature melanosomes in melanocytes.
It is caused by mutations in the gene coding for the myosin-5a (MYO-VA), or in the one coding for RAB27A.
Patients with mutations in RAB27A are associated to partial albinism with a defect in cytotoxic lymphocytes, and a syndrome of uncontrolled activation of macrophages and T lymphocytes (hemophagocytic syndrome), often fatal, and can only be treated with a bone marrow transplantation.
Patients with defects of myosin 5, however, associate to hypopigmentation an early and serious neurological deterioration, without immunological abnormalities.