The vas deferens is a tube-like structure in the male reproductive system that connects the epididymis, the site of storage of sperm, to the urethra, the tube that expels the sperm. Congenital absence of the vas deferens (CAVD) is a condition where the vas deferens fails to develop normally, resulting in defects in the absence of transport of the sperm to the urethra. The condition can be unilateral or bilateral; in the latter case, leading to azoospermia and infertility. From 65-90% of patients affected by this condition are carriers of cystic fibrosis, and mutations in the CFTR gene in these patients are actually causal of CBAVD. In fact, CBAVD is one of the most consistent features of CF. In most patients with CAVD, the body and tail of the epididymis also may be absent; however, the head is invariably present. Spermatogenesis is usually normal, even though sperms cannot be detected in the semen.

About 2% of men with infertility are supposed to suffer from CBAVD. Semen in affected individuals contains little or no sperms, and is usually acidic and low in volume. Since spermatogenesis is normal, infertility can be treated in such patients with the use of techniques like microsurgical epididymal sperm aspiration (MESA) and intracytoplasmic sperm injection (ICSI). However, considering the risk of having progenies affected with CF, it is important before such a procedure to undergo genetic counseling and preferably to get the carrier status of the female partner checked out.

Mutations in the CTFR gene are responsible for the condition in a large majority of cases of CBAVD. About 50-83% of CAVD patients have at least one known CFTR mutation, while about 10% have two known CFTR mutations. In men with two mutations, one is usually mild. The most common of these mutations is D508, followed by 5T, R117H, R75Q, G542X, N1303, W1282X, G551A, and R347H.