 Acute promyelocytic leukemia (APL) is characterized by the t(15;17) cytogenetic abnormality leading to the expression of two fusion genes, PML/RARA and RARA/PML, and by its sensitivity to all-trans retinoic acid (ATRA) differentiating treatment.     ///    In vitro response to all-trans retinoic acid of acute promyelocytic leukemias with nonreciprocal PML/RARA or RARA/PML fusion genes..     ///    "These insertions lead to the formation of potentially functional, nonreciprocal, PML/RARA or RARA/PML fusion genes, providing the unique opportunity to investigate in a human noncell-line model the respective role of PML/RARA or RARA/PML in retinoid signaling. "    ///    "The two cases expressing PML/RARA presented with an immunostaining pattern typical of APL and a positive response to ATRA, whereas the APL case expressing only a RARA/PML fusion transcript exhibited an immunostaining pattern typical of non-APL cells, and was resistant to ATRA. "    ///    "Our results confirm that sensitivity to ATRA requires expression of PML/RARA and strongly correlates with immunostaining, and demonstrate that expression of RARA/PML alone is sufficient for a cytological APL phenotype, but does not confer sensitivity to ATRA.. "    ///    "Here, we report the in vitro response to ATRA of these two cases as well as of a third case presenting with submicroscopic insertion (15;17) and expressing exclusively PML/RARA, by morphological, functional, and immunological assays. "