MAPK3 and hypophosphatemia: MAPK regulation of FGF23 transcription is consistent with the recent findings that activating somatic mutations of RAS causes FGF23-mediated hypophosphatemia in humans [72] and that ERK1/2 activation is involved in FGFR-mediated FGF23 transcription in UMR-106 osteoblasts in vitro[40].