Among the growth factors known to have expression in embryonic kidney, FGF1, 2, 9, and 20 were found to promote proliferation of nephron progenitors in vitro.345 Global knockout of FGF9 and FGF20 alone or together led to nephron progenitor apoptosis and subsequent renal agenesis.346 Exogenous FGF9 or FGF20 is sufficient to maintain the stemness of MM or sorted nephron progenitors in vitro.346 However, FGF1 knockout mice, alone and in combination with FGF2 knockout, have no nephron progenitor defects,347 and FGF2-null mice348 have no renal defects. Here, FGF9 is linked to renal agenesis.