Case report Down syndrome (DS) or trisomy 21 is a constitutional chromosomal abnormality, which may be mosaic in 1% to 4% of cases. DS mosaic diagnosis is difficult because most patients have a normal phenotype and show no significant clinical abnormalities. Patients with DS have a higher risk of developing acute leukemia such as acute lymphoblastic leukemia (ALL). We report the case of a 19-year old woman with mosaic trisomy 21 and ALL.