Phosphorylated serine (pS) sequence (989-993) in human BRCA1-Associated Carboxyl-terminal Helicase (BACH1) that mediates specific interaction with the carboxyl-terminal domain (BRCT) of the Breast Cancer Gene 1 (BRCA1) protein, which is cell cycle regulated and is required for DNA damage–induced checkpoint control during the transition from G2 to M phase of the cell cycle