A research team found a novel function of FGF2 in mammalian testis. Although it has demonstrated that both GDNF and FGF2 are the self-renewal factor for spermatogonial stem cells (SSCs) in vitro, present study revealed that FGF2 acts to facilitate the differentiation of SSCs in vivo. The understanding of molecular mechanism regulating SSCs has potential for future applications for male infertility treatment.