Abstract Purpose: Loss of TGFβ signaling increases error-prone alternative end-joining (alt-EJ) DNA repair. We previously translated this mechanistic relationship as TGFβ and alt-EJ gene expression signatures, which we showed are anticorrelated across cancer types. A score representing anticorrelation, βAlt, predicts patient outcome in response to genotoxic therapy. Here we sought to verify this biology in live specimens and additional datasets. Experimental Design: Human head and neck squamous carcinoma (HNSC) explants were treated in vitro to test whether the signatures report TGFβ signaling, indicated by SMAD2 phosphorylation, and unrepaired DNA damage, indicated by persistent 53BP1 foci after irradiation or olaparib. A custom NanoString assay was implemented to analyze the signatures’ expression in explants. Each signature gene was then weighted by its association with functional responses to define a modified score, βAltw, that was retested for association with response to genotoxic therapies in independent datasets. Results: Most genes in each signature were positively correlated with the expected biological response in tumor explants. Anticorrelation of TGFβ and alt-EJ signatures measured by NanoString was confirmed in explants. βAltw was significantly (P < 0.001) better than βAlt in predicting overall survival in response to genotoxic therapy in The Cancer Genome Atlas (TCGA) pancancer patients and in independent HNSC and ovarian cancer patient datasets. Conclusions: Association of the TGFβ and alt-EJ signatures with their biological response validates TGFβ competency as a key mediator of DNA repair that can be readily assayed by gene expression. The predictive value of βAltw supports its development to assist in clinical decision making.