Ataxia telangiectasia and Rad3-related (also known as ATR) is a serine/threonine kinase member of the PI3K family that plays a role in breast cancer by activating in response to single-stranded DNA strands, which only occur as intermediates during DNA repair, most notably at stalled replication forks undergoing nucleotide excision repair (NER) or homologous recombination repair (HR). The gene discussed is ATR; the disease is breast cancer.