Component of the SOSS complex, a multiprotein complex that functions downstream of the MRN complex to promote DNA repair and G2/M checkpoint. In the SOSS complex, acts as a sensor of single-stranded DNA that binds to single-stranded DNA, in particular to polypyrimidines. The SOSS complex associates with DNA lesions and influences diverse endpoints in the cellular DNA damage response including cell-cycle checkpoint activation, recombinational repair and maintenance of genomic stability. Required for efficient homologous recombination-dependent repair of double-strand breaks (DSBs) and ATM-dependent signaling pathways (By similarity). Component of the SOSS complex, composed of SOSS-B (SOSS-B1/NABP2 or SOSS-B2/NABP1), SOSS-A/INTS3 and SOSS-C/INIP. SOSS complexes containing SOSS-B1/NABP2 are more abundant than complexes containing SOSS-B2/NABP1. Directly interacts with ATM, SOSS-A/INTS3 and RAD51. Interacts with INTS7 (By similarity). Localizes to nuclear foci following DNA damage. Foci formation is not cell-cycle dependent. Partial colocalization with RAD51 after ionizing radiation treatment (By similarity). Phosphorylated by ATM in response to DNA damage. Phosphorylation prevents degradation by the proteasome, hence stabilization of the protein and accumulation within cells (By similarity). Belongs to the SOSS-B family. SOSS-B1 subfamily.