CD13 is a pan-(MPO-positive) AML antigen, while it is expressed in some B-lineage neoplasms (ALL and PLL) and in T-linaege neoplasms at pro-thymic stage. Some reports have described the lack of this antigen in MPO-positive AML, and tried to regard such MPO-postive AML cases as an clinical entity. However, considering the easy induction or enhancement of the expression in in vitro culture, it is pertinent to interpret that the expression of CD13 in these "CD13-negative and MPO-positive AML" cases is marginally positive, which is readily induced or enhanced in expression in in vitro culture. It can, however, be pointed out that the CD13 expression in ex vivo-positive cases are significantly stronger than that in ex-vivo-negative in vitro-positive cases. Such consideration is necessary particularly in interpreting the results obtained after overnight transportation in commercial laboratories.