In recent years, a link between the transcriptional regulators of lineage-specific gene expression and progenitor proliferation control has emerged. A main exponent of this phenomenon is the CCAAT/enhancer binding protein (C/EBP) family of basic region-leucine zipper proteins. These transcription factors control the differentiation of a range of cell types, and have key roles in regulating cellular proliferation through interaction with cell cycle proteins. More recently, their position at the crossroads between proliferation and differentiation has made them strong candidate regulators of tumorigenesis, and C/EBPs have been described as both tumor promoters and tumor suppressors.