SP1 and breast cancer: Even more interesting is that both Sp1 and E2F1 proteins were present in the co-precipitated protein complex in T24T cells in the absence of the XIAP RING domain but not detectable in T24T cells in the absence of the XIAP BIR domains (Fig. 4c), indicating that XIAP interacts with Sp1 and E2F1 through BIR domains in BC cells.