Human semen not only accumulates genetic mutations with age; as the percentage of sperm carrying potentially serious mutations increases, so does the risk of developing diseases in offspring. The authors discovered that the male germ line is subject to positive selection. "We expected to find evidence that selection influences mutations in sperm," said Matthew Neville, coauthor of the study published this month in the journal Nature. "What surprised us was how much the number of sperm carrying mutations associated with serious diseases increases." The researchers estimated that about 3 to 5 percent of sperm from middle-aged and older men carry some potentially pathogenic mutation in the exome (the coding part of the genome).