PGR and breast cancer: It is worth noting that the highest expression of polyunsaturated lipids (i.e., PE(38:4), PE(P-38:5), PE(36:4), and C20:4) in BT-20 cell membrane from a primary tumor, along with a weakly invasive ability and negative for estrogen and progesterone receptors were observed relative to other five breast cancer cell lines derived from metastatic breast cancer tumors (Fig. 2D)20, indicating that the unsaturated degree of membrane lipids are closely associated with the types of breast cancer cells.