5q- syndrome is a rare disorder caused by loss of part of the long arm (q arm, band 5q31.1) of human chromosome 5. This syndrome affects bone marrow cells, causing treatment-resistant anemia, and myelodysplastic syndromes that may lead to acute myelogenous leukemia. Examination of the bone marrow shows characteristic changes in the megakaryocytes. They are more numerous than usual, small and mononuclear. There may be accompanying erythroid hyperplasia.

Lenalidomide is a new agent that is showing promise in treating 5q- syndrome.