Sorafenib is a potent inhibitor of multiple oncogenic, stromal and angiogenic receptor tyrosine kinases. Germline variants in VEGF-pathway genes and in sorafenib pharmacology genes might associate with prognosis and/or sorafenib efficacy in metastatic renal cell carcinoma (mRCC patients). A total of 295 mRCC patients from the phase III TARGET trial were genotyped using candidate germline variants from 56 candidate genes implicated in angiogenesis, sorafenib pharmacology and/or RCC prognosis/pathogenesis. Seven variants that significantly associated with overall survival (OS) in mRCC patients treated with sorafenib, and an additional two variants associated with OS in a combined analysis of both treatment arms. Statistical associations between genetic variants and outcomes in cancer studies should be supported with molecular mechanistic evidence of variant function to aid in biomarker validation. Variants identified in Aim 1 that significantly associated with OS were analyzed using in silico bioinformatic tools to prioritize in vitro validation assays. Cell viability assays validated one non-synonymous variant in FLT-4, and dual reporter gene luciferase assays validated two intronic VEGFA variants in three different cell lines. Novel pathways and targets of sorafenib activity remain to be identified. Primary mouse embryonic fibroblasts (MEFs) from 32 inbred strains were profiled for sorafenib cytotoxicity utilizing high content imaging and simultaneous evaluation of cell health parameters (cell viability, membrane permeability, mitochondrial membrane potential, and cytochrome C release). One quantitative locus (QTL) on chromosome 9, which reached genome-wide significance and significantly associated with cytochrome C release, was identified. A total of nine genes, expressed in MEF cells at mRNA level, were present in this QTL. A second QTL associated with cell viability was also identified. A total of 13 candidate genes, expressed in MEF cells at mRNA level, were present in this QTL. In the future, functional validation of candidate genes under these two identified QTLs, using knockdown and overexpression approaches, will be conducted in MEF and human cell lines.