Required for checkpoint signaling after DNA damage. Required for ATR expression, possibly by stabilizing the protein (By similarity). Heterodimer with ATR. The heterodimer binds the RPA complex and is then recruited to single-stranded DNA. Interacts with CEP164 (via N-terminus). Interacts with CINP (By similarity). Interacts with ATR (By similarity). Redistributes to discrete nuclear foci upon DNA damage. The EEXXXDDL motif is required for the interaction with catalytic subunit PRKDC and its recruitment to sites of DNA damage. Phosphorylated by ATR. Belongs to the ATRIP family.