FAP and breast cancer: We then exposed normal mouse 3T3 fibroblasts to CM from mouse 4T1 BC cell cultures and found a significantly induced expression of α-smooth muscle actin (α-SMA) and fibroblast activation protein (FAP), both markers of the myofibroblast-activated phenotype of CAFs, a result that we replicated in human WI-38 fibroblasts treated with CM from 231 BC cells (Figure 2C), therefore supporting the role of BC-secreted factors in the acquisition of the CAF phenotype by normal fibroblasts, i.e., in a heterologous context.