Penetrance is usually age related, meaning that the trait is not expressed in most carriers at birth but occurs with increased frequency as the carriers get older. For example, germline mutations in mismatch repair genes associated with hereditary nonpolyposis colorectal cancer (HNPCC) are incompletely penetrant. So not all individuals who carry these mutations will get colorectal cancer, but the risk increases as individuals age. About 20 percent of carriers will never develop colorectal cancer.