Similarly, pharmacological doses of ATRA exhibit activity against non-APL AML cells in liquid culture; however, stromal cells, which also up-regulate their CYP26 expression upon exposure to ATRA, blocked the pro-differentiation effect of ATRA on APL and AML cell lines as well as CD34+CD38−ALDHint leukemic cells from primary samples, which were rescued by CYP26 inhibition [98]. This evidence concerns the gene CD34 and acute promyelocytic leukemia.