The effect of all-trans retinoid acid (ATRA) on the expression of Notch1 gene by real-time reverse transcriptase polymerase chain reaction (RT-PCR) in acute promyelocytic leukemia cells (APL), NB4, and HL-60 lacking t(15;17) was studied. The cells were treated with ATRA 0.5 microM for up to 96 hours. The increased transcript level of Notch1 was in concert with that of CD11b in NB4 cells, but not in HL-60 cells. The expression of Notch1 gene might be specific for APL cells. As Notch1 gene is involved in the differentiation and leukemogenesis in lymphoid neoplasm, observations suggest that Notch1 is involved in ATRA-modulated differentiation process in APL.