We found that PEDF mRNA and protein levels were dramatically reduced in endocrine-resistant MCF-7:5C, MCF-7:2A, and BT474 breast cancer cells compared with endocrine-sensitive MCF-7, T47D, and ZR-75-1 cells, and that loss of PEDF was associated with enhanced expression of pSer167ERα and the receptor tyrosine kinase rearranged during transfection (RET). Here, NTRK1 is linked to breast carcinoma.