Detects endogenous levels of proteins containing the phospho-(Ser/Thr) ATM/ATR substrate motif. Preferentially binds peptides/proteins that contain phospho-Ser/Thr preceded by Leu or similar hydrophobic amino acids at the -1 position and followed by Gln at the +1 position. No cross-reactivity is observed with corresponding non-phosphorylated sequences or with other phospho-Ser/Thr-containing motifs. Species cross-reactivity: All species.