CD44 and breast cancer: Since a splice variant of CD44 [62] was reported to interact with and stabilize xCT, which was responsible for uptake of a radiotracer (S)-4- (3- 18F-Fluoropropyl)-L-Glutamic Acid (18F-FSPG), the correlation between tumor uptake of 18F-FSPG and system xC/CD44 were examined in a few mini-cohorts of patients with NSCLC, breast cancer, hepatocellular carcinoma and prostate cancer [63–65].