The explanation relies upon two considerations: (i) when looking selectively to the B cell compartment, CD39 expression and ATPase/ADPase activity were found to be quite similar in leukemic cells and B cells collected from healthy donors and (ii) in healthy people the T cell fraction represents the majority of circulating lymphocytes population, at variance with CLL patients. The gene discussed is ENTPD1; the disease is B-cell chronic lymphocytic leukemia.