Up-regulation of CD38 expression upon IL-2 exposure according to genotype. Peripheral blood mononuclear cells from 40 CLL patients (21 CC, 17 GC, and 2 GG) were cultured for 72 hours in the presence or absence of IL-2 (100 IU/mL) before staining for surface CD38 and CD19. A MFI ratio was determined as follows: MFI levels after IL-2 exposure/MFI levels after culture in complete medium. G allele carriers showed a significantly higher surface CD38 expression in response to IL-2, compared with CC homozygotes (P = .028).