MSX2 and hydrops fetalis: Many identified genes of the K4 clusters have been reported as a key regulator of HF differentiation and cycling in human or mouse, such as BARX2 [37], FOXN1 [38], OVOL1 [39], and VDR [40–42], DLX3 [43], HOXC13 [44, 45], MSX2 [46, 47], and MSX1 [48] in cluster K4.