The present invention relates to identification of the consensus sequence phosphorylated by ATM kinase. This, in turn, permitted identification of ATM kinase target proteins, and development of a convenient assay system for ATM kinase phosphorylation using fusion polypeptides as substrates. The assay system is adaptable to screening for ATM modulators, particularly inhibitors. In a specific embodiment, the substrate recognition sequence and mutagenized variants of this sequence were incorporated in a GST fusion protein and assayed for phosphorylation by ATM kinase. This assay system is useful in screening for ATM inhibitors. ATM function assays were validated using an ATM-kinase dead dominant-negative mutant.