Background: Mutational activation of the RAS pathway occurs in >50% of human CRC tumors, and to date has been difficult to overcome. We recently identified receptor tyrosine phosphatase-S (PTPRS) as an important regulator of the RAS pathway, which is mutated in ~11% of CRC. Methods: Global gene expression and DNA sequencing data (1321 cancer genes) were integrated across 468 CRC tumors to identify PTPRS as one of the most commonly mutated genes with a high correlation to a RAS activation score. Apoptotic responses to inhibitors of ERK, EGFR and their combination were tested in isogenic wild-type and mutant RAS cell lines where PTPRS had been knocked out by CRISPR. Results: PTPRS was in the top 15 genes correlating with RAS pathway activation scores; as expected, KRAS, BRAF were the most correlated genes. In order to determine the effect of the loss of PTPRS on RAS/ERK signaling, we used CRISPR to knockout PTPRS in an isogenic pair of CRC cell lines (HCT116) with mutant or wild type KRAS. Cell lines lacking PTPRS expression had elevated phospho-ERK activity. Interestingly, cell lines with PTPRS knockout and elevated ERK phosphorylation had an increased sensitivity to ERK inhibitors. Moreover, the loss of PTPRS allowed ERK to localize to the nucleus. Apoptosis increased dramatically from 13.1% in control cells to 53.8% in PTPRS KO cells treated with both ERK and EGFR inhibitors (Table), a synergistic effect not seen in wild type KRAS cells, where only ERK inhibitors were active. Conclusions: The loss of PTPRS allows increased activation of ERK (and the RAS pathway) which is associated with ERK addiction and increased sensitivity to ERK inhibitors. Surprisingly, ERK inhibitors synergize with EGFRi in wild type KRAS cell lines absent PTPRS. These data suggest the potential for mutant PTPRS to serve as a biomarker for ERK/EGFR inhibition.