Background: Mutation-derived tumor antigens (MTAs) arise as a direct result of somatic variations that occur during carcinogenesis and can be characterized via genetic sequencing and used to identify MTAs. We developed a platform for a fully-personalized MTA-based vaccine in the adjuvant treatment of solid and hematological malignancies. Methods: This is a single-arm, open label, proof-of-concept phase I study designed to test the safety and immunogenicity of Personalized Genomic Vaccine 001 (PGV001) that targets up to 10 predicted personal tumor neoantigens based on patient’s HLA profile (ClinicalTrials.gov: NCT02721043). Results: Patients who completed vaccination with PGV001_002 (head and neck squamous cell cancer) received 10 doses of vaccine comprising 10 long peptides (LP) combined with poly-ICLC (toll-like receptor-3 agonist) intradermally. Vaccine-induced T-cell responses were determined at weeks 0 and 27 (before and after treatment, respectively), ex vivo by interferon (IFN)-g enzyme-linked immunospot assay and after expansion by intracellular cytokine staining. Overlapping 15-mer peptides (OLPs) spanning the entirety of each LP and 9-10-mer peptides corresponding to each predicted class I epitope (Min) were pooled. Ex vivo responses to these peptide pools were undetectable at week 0 but were evident at week 27 against 2 OLPs out of 10 (20%) and in 5 Min out of 10 (50%). After in vitro expansion, neoantigen-specific CD4⁺ and CD8⁺ T-cell responses were found in 5 out of 10 pooled peptides (50%). 7 out of 10 (70%) epitopes elicited polyfunctional T-cell responses (secretion of INF-g, TNF-a, and/or IL-2) from either CD4⁺ or CD8⁺ T cells. Conclusions: The PGV001 vaccine in our first patient showed both safety and immunogenicity, eliciting CD4⁺ and CD8⁺ responses to the vaccine peptides. As we enroll additional patients in this clinical trial, and perform deeper phenotyping of their tumor-reactive T cells, we will learn the determinants necessary for the successful generation of MTA-based vaccines, while informing future immunotherapeutic approaches and rational combinations. Clinical trial information: NCT02721043