Monarch Disease Ontology id MONDO_0018935 (hairy cell leukemia) (HCL) is a rare type of leukemia in which abnormal B-lymphocytes are present in the bone marrow, spleen and peripheral blood. It is a slowly progressive chronic lymphocytic leukemia (CLL). The name comes from the abnormally shaped lymphocytes with hair-like projections. Also known as: HCL, HCL-C, leukemic reticuloendotheliosis.