A rare case of acute megakaryoblastic leukaemia with constitutional ring chromosome 21.

Constitutional ring chromosome 21 is a rare genetic abnormality which usually arises early in embryogenesis. There is breakage with the deletion of genetic material from both ends of chromosome 21, followed by joining of the ends to form a ring.1 The quantity of genetic material lost is highly variable and accounts for the varied phenotypic expression. Affected children may be asymptomatic, or may have intellectual disability, abnormal facies, infertility, microcephaly and failure to thrive. The clinical phenotype may resemble other disorders of chromosome 21, including Down syndrome.1 Acute megakaryoblastic leukaemia is frequently associated with Down syndrome or Down syndrome mosaicism, but the association with ring chromosome 21 is very rare. To the authors' knowledge, constitutional ring chromosome 21 has been reported in only three cases of acute lymphoblastic leukaemia and three cases of acute myeloid leukaemia.