Forkhead box protein P1 (FOXP1) is a transcriptional repressor. It plays an important role in the specification and differentiation of lung epithelium. Can act with CTBP1 to synergistically repress transcription but CTPBP1 is not essential. Essential transcriptional regulator of B cell development. FOXP1 protein plays an important role in the specification and differentiation of lung epithelium, also acts cooperatively with FOXP4 to regulate lung secretory epithelial cell fate and regeneration by restricting the goblet cell lineage program. FOXP1 is useful in subclassification of DLBCL and a high cutoff (≥80%) for FOXP1 is needed to achieve high specificity for the ABC subtype.