The transcription factor C/EBPα is more potent than C/EBPβ in inducing granulocitic differentiation and inhibiting BCR/ABL-expressing cells. We took a "domain swapping" approach to assess biological effects, modulation of gene expression, and binding to C/EBPα-regulated promoters by wild-type and chimeric C/EBPα/C/EBPβ proteins. Wild-type and N-C/EBPα+ C/EBPβ-DBD induced transcription of the granulocyte-colony stimulating factor receptor (G-CSFR) gene, promoted differentiation, and suppressed proliferation of K562 cells vigorously; instead, wild-type C/EBPβ and N-C/EBPβ+C/EBPα-DBD had modest effects, although they bound the G-CSFR promoter like wild-type C/EBPα and N-C/EBPα+C/EBPβ-DBD. Chimeric proteins consisting of the TAD of VP16 and the DBD of C/EBPα or C/EBPβ inhibited proliferation and induced differentiation of K562 cells as effectively as wild-type C/EBPα. Gene expression profiles induced by C/EBPα resembled those modulated by N-C/EBPα+C/EBPβ-DBD, whereas C/EBPβ induced a pattern similar to that of N-C/EBPβ+C/EBPα-DBD. C/EBPα activation induced changes in the expression of more cell cycle- and apoptosis-related genes than the other proteins and enhanced Imatinib-induced apoptosis of K562 cells. Expression of FOXO3a, a novel C/EBPα-regulated gene, was required for apoptosis but not for differentiation induction or proliferation inhibition of K562 cells.