CHOPCHOP is an online web tool designed to facilitate the design of CRISPR/Cas9 and TALEN target sites for genome editing. It allows users to input gene identifiers, genomic regions, or DNA sequences and provides an interactive visualization of the gene with candidate target sites displayed at their genomic positions and color-coded according to quality scores. The tool uses efficient sequence alignment algorithms to minimize search times and rigorously predicts off-target binding of single-guide RNAs (sgRNAs) and TALENs. It also provides restriction sites and primer candidates for each possible target site, enabling a streamlined pipeline for mutant generation and validation. CHOPCHOP is user-friendly, fast, and integrates primer design and restriction site visualization, making it a valuable tool for genome engineering. It offers a dynamic graphical output display with interactive visualization of the gene, and provides downloadable results including GenBank files with annotations of the gene's exons, introns, and target sites. CHOPCHOP is distinct from other tools due to its wide range of inputs, integrated TALEN and CRISPR/Cas9 target design, and comprehensive off-target prediction. It is a valuable resource for genome editing technologies, providing an efficient and intuitive way to select optimal target sites for CRISPR/Cas9 and TALEN-based mutations.CHOPCHOP is an online web tool designed to facilitate the design of CRISPR/Cas9 and TALEN target sites for genome editing. It allows users to input gene identifiers, genomic regions, or DNA sequences and provides an interactive visualization of the gene with candidate target sites displayed at their genomic positions and color-coded according to quality scores. The tool uses efficient sequence alignment algorithms to minimize search times and rigorously predicts off-target binding of single-guide RNAs (sgRNAs) and TALENs. It also provides restriction sites and primer candidates for each possible target site, enabling a streamlined pipeline for mutant generation and validation. CHOPCHOP is user-friendly, fast, and integrates primer design and restriction site visualization, making it a valuable tool for genome engineering. It offers a dynamic graphical output display with interactive visualization of the gene, and provides downloadable results including GenBank files with annotations of the gene's exons, introns, and target sites. CHOPCHOP is distinct from other tools due to its wide range of inputs, integrated TALEN and CRISPR/Cas9 target design, and comprehensive off-target prediction. It is a valuable resource for genome editing technologies, providing an efficient and intuitive way to select optimal target sites for CRISPR/Cas9 and TALEN-based mutations.