One in 25 men carry a cystic fibrosis gene mutation that can cause defects in the vas deferens leading to reduced sperm count.

“1 in 25 men will carry just one copy and therefore will not have any symptoms of cystic fibrosis. But those 1 in 25 men will have defects in the architecture of the vas deferens, the duct through which the ejaculate needs to pass in order to eventually be ejaculated out of the urethra. And so what that means is that these men can have what appears to be normal semen volume but that they won't have normal numbers of sperm.”