As we have seen, cancer arises from the accumulation of multiple mutations in a single cell. The rate at which mutations occur is affected not only by the rate at which they arise, but also by the efficiency with which errors are corrected by DNA-repair systems (see Chapter 13). Defects in genes that encode components of these repair systems have been consistently associated with a number of cancers. People with xeroderma pigmentosum, for example, are defective in nucleotide-excision repair, an important cellular repair system that normally corrects DNA damage caused by a number of mutagens, including ultraviolet light. Likewise, about 13% of colorectal, endometrial, and stomach cancers have cells that are defective in mismatch repair, another major repair system in the cell.