- Macroorchidism in FMR1 knockout mice is caused by increased Sertoli cell proliferation during testicular development. Slegtenhorst-Eegdeman, K.E., de Rooij, D.G., Verhoef-Post, M., van de Kant, H.J., Bakker, C.E., Oostra, B.A., Grootegoed, J.A., Themmen, A.P. Endocrinology (1998)