FOXP1 (forkhead box protein P1) is a member of the forkhead box (FOX) subfamily P family of proteins. FoxP1 functions as a transcriptional repressor and plays an important role in the specification and differentiation of lung epithelium. It can act with CTBP1 to synergistically repress transcription but CTPBP1 is not essential. Additionally, FOXP1 is an essential transcriptional regulator of B cell development [taken from the Universal Protein Resource (UniProt) www.uniprot.org/uniprot/Q9H334].