CD44 and breast cancer: By exogenously treating MDA-MB-231 breast cancer cells with recombinant caspases-3 and -7 in different pH environments, we have shown that caspases-3 and -7 can cleave NRP-1 and CD44 in both normal and acidic pH, while CSPG4, which was identified as a unique caspase-3 substrate, was expectedly cleaved only in a neutral-pH environment (Figure 5).