CHEK2 is a gene linked to Breast cancer. The CHEK2 protein is thought to prevent cancer by inhibiting the division of cells that have damaged DNA.
SNPs in CHEK2 include:
- rs17879961, known as I157T
- i4000462 in 23andMe reports, known as 1100delC, has been associated with a 1.4x to 4.7x increased risk for breast cancer. Carriers of one copy of this deletion produce half the normal amount of the CHEK2 protein. This deletion is one of the more common genetic variants associated with breast cancer.
"The aim of this study was to determine whether common polymorphic variants in CHEK2 are associated with an increase in breast cancer risk. We examined two variants: an insertion of a single nucleotide in intron 1 rs3841692 and a single nucleotide a to g substitution rs1805129, which is a silent alteration in codon 84 of the coding sequence." 
[PMID 17517688] Uncommon CHEK2 mis-sense variant and reduced risk of tobacco-related cancers: case-control study.
[PMID 17508290] Genetic susceptibility to breast cancer.
[PMID 17458694] Detection of the CHEK2 1100delC mutation by MLPA BRCA1/2 analysis: a worthwhile strategy for its clinical applicability in 1100delC low-frequency populations?
[PMID 17428325] Common variants in the ATM, BRCA1, BRCA2, CHEK2 and TP53 cancer susceptibility genes are unlikely to increase breast cancer risk.
[PMID 17428320] Identification of women with an increased risk of developing radiation-induced breast cancer: a case only study.