Plays a role in DNA damage response (DDR) signaling upon genotoxic stresses such as ionizing radiation (IR) during the S phase. Recruited to sites of DNA damage through interaction with the 9-1-1 cell-cycle checkpoint response complex and TOPBP1 in a ATR-dependent manner. Required for the progression of the G1 to S phase transition. Plays a role in the stimulation of CHEK1 phosphorylation. Interacts with RAD9A, RAD18, TOPBP1 and UBE2N. Localizes to sites of DNA damage in a H2AX-independent manner. Weakly expressed in testis, prostate, ovary, thymus and small intestine. Expressed strongly in breast cancer cells. Up-regulated in breast cancer cells.