ATR and Ataxia-telangiectasia: In an attemptto identify the human ortholog of rad3, Tony Carr’s laboratory useddegenerated PCR based on the sequences of Rad3 and Mec1 and subsequently screened acDNA library isolating the coding region of ATR(ataxia telangiectasia and rad3-related) (Bentley etal., 1996) (Figure 1).At the same time, Cimprich et al.(1996) relied on an expressed sequence tag (EST) with sequence similarityto the PI3K-related kinases FRAP, Tor1p and Tor2p to isolate the cDNA correspondingto FRP1 (FRAP-relatedprotein 1).