PGF and breast cancer: The score is derived from tumor expression of docking protein-4 (DOK4), holocytochrome c synthase (HCCS), placental growth factor (PGF), and SHC SH2 domain-binding protein-1 (SHCBP1) genes, which were identified based on differential mRNA expression analysis of a human breast cell line and its metastatic variant cells, and with the clinical outcome data of breast cancer patient cohorts.