We hypothesize that cyst formation in the male genital tract of BBS patients results from a physio-pathological mechanism similar to that described in polycystic kidney disease: cyst formation would result from a dysfunction of the dimer polycystin 1–polycystin 2 (PC1-PC2) (11, 41, 42), inducing excess cell growth, proliferation, and secretion. The gene discussed is PKD2; the disease is polycystic kidney disease.