Cytogenetics and FISH negative cryptic acute promyelocytic leukemia with CD56 expression.

Acute promyelocytic leukemia (APL) is characterized by reciprocal translocation t(15;17)(q22;q21) and has a favorable prognosis upon immediate recognition and treatment. However, rare cases of APL show a cryptic insertion of retinoic acid receptor alpha (RARA) gene into promyelocytic leukemia (PML) gene which is negative both by fluorescence in situ hybridization (FISH) and conventional cytogenetics (CC). Morphology, cytochemistry and flow cytometry play a key role in early identification of such cases. Polymerase chain reaction (PCR) remains the most efficient diagnostic modality for detection of cryptic APL and other variants. It is important to identify these cases as they show beneficial response to retinoids and favourable prognosis. We herein present a rare case of cryptic APL negative by FISH and conventional cytogenetics but positive for PML-RARA by PCR.