Immature myeloid cells derived from the spleens of hCG–PML-RARα mice are sensitive to ATRA. (A and B) S1 nuclease protection assays using total RNA purified from total splenocytes cultured in the presence or absence of 1 μmol/L ATRA as indicated. Positions of probe bands protected by correctly spliced endogenous mCG or β₂ microglobulin mRNAs are shown. Levels of mCG transcripts from nonleukemic (A) or leukemic (B) mice both declined substantially within 24 hours of ATRA treatment. Splenocytes from the nontransgenic littermates of these mice did not have any detectable mCG mRNA at any point during the course of the experiments, as expected. (C and D) Splenocytes from a leukemic mouse (FL no. 135) expressing the hCG–PML-RARα transgene incubated in the absence (C) or presence (D) of 1 μmol/L ATRA for 48 hours. Large clusters of immature myeloid cells (large arrows in [C]) involute in the presence of ATRA (D, large arrow). A macrophage laden with possible nuclear fragments is invading the apoptotic cell cluster (black arrow head). Several cells with fragmented, apoptotic nuclei are seen in the ATRA-treated culture (small arrows).