Analysis of cloned HL-60 cell lines conditionally expressing c-Maf. (A) Lineage analysis of stable HL-60 lines expressing c-Maf. Cell lines containing either vector control (CB6, top row) or c-Maf (c-Maf, middle row) were untreated (none) or incubated with 150 μmol/L ZnSO₄ (Zn⁺²), or parental HL-60 cells (bottom row) were untreated (none) or TPA treated (TPA). Cytospin preparations were stained with Wright-Giemsa (left two panels) or -napthyl butyrate esterase monocyte-specific stain (right two panels). (B) Conditional expression of c-Maf over time. Individual HL-60 clonal cell lines containing either CB6 vector (HL-60 CB6 clone 3) or CB6-c-Maf-(HL-60 c-Maf clones 4 and 6) were induced with ZnS0₄ for the indicated time periods and lysates were probed for c-Maf expression with an antiserum recognizing the c-Maf protein. (C) Induction of c-Maf alters normal growth kinetics. Viable cell numbers were determined for vector control-containing (CB6) or c-Maf–containing (c-Maf clones 4 and 6) stable cell lines by trypan blue dye exclusion at the indicated intervals after zinc exposure. Representative data from 2 individual experiments are presented.