The gene for cystic fibrosis is positioned very close to the gene that can be responsible for men being born without a vas deferens (the tube connecting the testicle to the penis), and the conditions are closely associated. Hence men who are found to produce no sperm should be tested to see if they are carriers of cystic fibrosis. Women who have cystic fibrosis can have very thick cervical mucus, just as they have thick bronchial mucus, and therefore they can be subfertile because the sperm have difficulty swimming through it. However, female carriers of cystic fibrosis are not thought to have a fertility problem.