Morphologic and lineage analysis of cloned U937 cell lines conditionally expressing c-Maf. Cytospin preparations of parental U937 cells either untreated (top row) or treated with TPA (second row); or U937 clonal lines containing either vector control (U937 CB6, third row) or c-Maf expression constructs (U937 c-Maf, bottom row) incubated for 48 hours with 150 μmol/L ZnSO₄. Left column: Wright-Giemsa stain for morphological examination; right column: -napthyl butyrate esterase stain specific for monocytic lineage.