The forkhead associated (FHA) domain-containing protein Smad nuclear interacting protein 1 (SNIP1) has multiple cellular functions, including the ability to interact with DNA-binding transcription factors and transcriptional coactivators. Moreover, we have demonstrated previously that SNIP1 regulates cyclin D1 expression and promoter activity. Here, we identify a new function for SNIP1 as a regulator of ATR checkpoint kinase-dependent pathways in human U-2 OS osteosarcoma cells: SNIP1 is required for p53 induction in response to ultraviolet light treatment and selectively regulates the phosphorylation of known ATR target proteins, including p53, Chk1 and the histone variant H2AX. These activities are independent of its ability to regulate cyclin D1 expression. Significantly, SNIP1 is also required for ATR-dependent functions of the human p14(ARF) tumour suppressor, including its ability to modulate the activity of the RelA(p65) NF-kappaB subunit. This, together with its other described functions, suggests that SNIP1 could have an important role during tumorigenesis and cancer therapy.