Another rare cause is a mutation in the gene for the beta-subunit of FSH. A man with this mutation had delayed puberty with low serum testosterone and FSH concentrations, but a high serum LH concentration. The low serum testosterone concentration is hard to explain because the Leydig cells do not have FSH receptors. However, this finding may indicate that normal human Leydig cell function is dependent on normal function of Sertoli cells, which do have FSH receptors