The FOXP subfamily is defined by four members (FOXP1–FOXP4). FOXPs function as transcription factors and possess distinct transcriptional regulatory and DNA-binding domains. The N terminus of FOXPs contains a transcriptional repressor region with a zinc-finger/leucine zipper motif. The DNA-binding domain of FOXPs is uniquely positioned among forkhead proteins near the C terminus. FOXP1 is widely expressed in human tissues and regulates the development of many tissues, including heart, thymus, and lung. FOXP1-deficient embryos have severe defects in cardiac morphogenesis, including outflow tract septation and cushion defects, resulting in embryonic death.