Known BC susceptibility genes such as BRCA1, BRCA2, ATM, CHK2 and TP53 are involved in maintaining the genomic stability through their involvement in DNA double-strand break repair, transcription-coupled repair and homologous recombination; defects in their expression are expected to lead to an increased mutagen sensitivity as expression of an intermediate phenotype in cancer development (Venkitaraman, 2002, 2009; Smith et al, 2003; Hsu et al, 2007; Ralhan et al, 2007; Couch and Wang, 2009; Kwei et al, 2010; Latimer et al, 2010). The gene discussed is ATM; the disease is breast cancer.