The incidence of most cancers is significantly lower among people with Down syndrome as compared to the general population, and researchers believe they have figured out why - people with the congenital(生まれつきの)disorder have extra copies of a gene that helps prevent tumors from developing the blood vessels that nourish them. Individuals with Down syndrome have an extra copy of chromosome 21, giving them an extra version of each of the 231 genes found on that chromosome. One of these genes codes for(特定のアミノ酸をしている遺伝情報を持つ)a protein that suppresses vascular(血管の)endothelial(細胞の内覆組織)growth factor, a compound necessary for the development of blood vessels.