Recent seminal work by the group of Ehrenfeld et al. has shown that the kinin-generating kallikrein KLK1 and its substrates, kininogens, are expressed in estrogen-sensitive (MCF-7, T47D, ZR-75–1) and estrogen-insensitive (MDA-MB-231) BC cells [68]. This evidence concerns the gene KLK4 and breast cancer.