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Antigen processing results in the formation of different epitopes, which bind to specific HLA molecules and are presented to specific cells in our organism. When an antigen is recognized in the human organism, the immune system is activated, and a well-regulated process initiates. The same surveillance occurs when part of a pathogen is used as a component of therapeutic tools, as in CRISPR/Cas9. And it may result in the death of edited cells. The most frequent HLA alleles in Brazil (HLA-A*02:01, in the class-I pathway; HLA-DRB1*03:01, HLA-DRB1*07:01, and HLA-DQB1*03:01 in the class-II pathway) were analyzed. Curated sequences for S. pyogenes serotype M1 (ID: Q99ZW2), S. thermophilus (ID: G3ECR1), and S. aureus (ID: J7RUA5) were obtained from UniProt. The epitope predictions followed the natural pathway for antigens presentation in class I pathway nucleated cells, using MHC-I Processing predictor and MHC-I Binding Predictions, both available in the IEDB database. The class II epitopes were predicted through TepiTool, also available in the IEDB database. IEDB has been checked for MHC binding or IFN/γ positive assays for all results. Our results show that, for the most commonly used Cas9, several types of epitopes can be generated that would predictably elicit an immune response by particular HLA alleles common in Brazil. For 15 epitopes there are reports in IEDB's previous works. Indeed, for the GLFGNLIAL, ILEDIVLTL, and NLIALSLGL, (S. pyogenes) there are positive results in in vitro tests for IFN/γ and/or MHC binding. That suggests a real possibility of immune response hampering the efficiency of gene editing using these Cas9. Our results are the first to suggest a real possibility of immune response hampering gene editing efficiency using these Cas9 in the Brazilian population with these particular HLA alleles. Therefore, mechanisms to avoid the immune system may be necessary to increase the success of gene editing. [ABSTRACT FROM AUTHOR]