To assess whether the observed inhibition of ascites-derived BC-3 and BCBL-1 cell proliferation resulted from apoptosis, ex-vivo treated BC-3 and BCBL-1 cells were stained with Annexin V. A moderate increase in the apoptotic population was detected upon treatment with single agent arsenic or IFN but a major increase in Annexin V positive cells exceeding 50% at 48h was observed for both ascites-derived BC3 and BCBL-1 cells treated with either arsenic/IFN or AZT/IFN (p< 0.001 for both) (Figure 3B, Figure S3A). This evidence concerns the gene IFNA1 and breast cancer.