Down syndrome (DS), or Trisomy 21, is defined by an extra copy of chromosome 21 in all cells. In humans there are normally 2 copies of each of 23 chromosomes for a total of 46 in each cell; in DS there are 47.

An extensive review of literature available in 1998 showed that the distribution of cancer in DS in quite different than in the general population (GP). A number of epidemiological studies identified then, or published since, have confirmed that DS is associated with a marked increased risk of leukemia. Increased risks of incidence of or mortality from lymphomas and a few solid tumors have also been reported. On the other hand, a number of studies have suggested that other solid tumors are less common in persons with DS than in the general population. A potential DS-related protective effect against some cancers has been suggested, and this has encouraged a number of studies of a possible genetic mechanism of tumor suppression related to DS.

Evidence on which tumors may be over- or under-represented in DS is mixed. Ten studies reviewed in Sasco et al.