Intense non-specific esterase activity (NSE) [naphthyl acetate esterase, naphthol AS-D acetate esterase (CAE), and alpha-naphthyl butyrate esterase] represents one of the most specific hallmarks of cytochemical staining in monocytic lineage leukemia, resulting strongly positive in both pathological monoblasts and promonocytes, even if 10–20% of AMoL cases show NSE negativity or weak positivity. This evidence concerns the gene ENO2 and leukemia.