The vas deferens, or rather the absence of the tube, is sometimes a natural cause of sterility in men. A condition known as congenital bilateral absence of the vas deferens (CBAVD) is responsible for approximately two to five percent of cases of male infertility. Techniques are available that assist men with CBAVD to have their own biological children, but testing is advised before such action is taken. A large majority of those suffering from CBAVD have a mutation in at least one of the genes that is associated with cystic fibrosis. Therefore, individuals with CBAVD that use assisted reproductive techniques are more likely to produce a child that exhibits the recessive disorder.