Morphology and Bcl-x expression in HL-60 cells treated with PMA or RA. HL-60 cells untreated or after culturing in the presence of PMA for 4 days or RA for 6 days were cytocentrifuged and either stained with May-Grunwald-Giemsa solution or analyzed for the expression of Bcl-x using a rabbit anti–-Bcl-x antibody. After incubation with biotinilated goat antirabbit IgG and ExtrAvidin alkaline phosphatase, staining was developed using BCIP/NBT substrate (original magnification × 1,000). No signal was detectable in the negative controls. All stainings are from a representative experiment (n = 3).