Representative flow cytometric histograms show CD38 expression in RAR-α (A-C) and RXR-α (D-F ) –transfected HL-60R cells, incubated in medium alone (A and D) or in medium containing 10 nmol/L ATRA (B and E) or 10 mmol/L 9-cis RA (C and F ) overnight. The cells were stained with either isotypic control IgG₁ or anti-CD38 antibody (Leu-17) as described in the Materials and Methods.