CHOPCHOP is an online tool designed to facilitate the selection and design of targeting constructs for genome editing using CRISPR/Cas9 and TALEN methods. It accepts various inputs, including gene identifiers, genomic regions, and pasted sequences, and provides advanced options for target selection. CHOPCHOP uses efficient sequence alignment algorithms to minimize search times and rigorously predicts off-target binding of single-guide RNAs (sgRNAs) and TALENs. The tool generates an interactive visualization of the gene with candidate target sites, color-coded according to quality scores, and visualizes restriction sites and primer candidates for streamlined mutant generation and validation. CHOPCHOP's ease of use and speed make it a valuable resource for genome engineering.CHOPCHOP is an online tool designed to facilitate the selection and design of targeting constructs for genome editing using CRISPR/Cas9 and TALEN methods. It accepts various inputs, including gene identifiers, genomic regions, and pasted sequences, and provides advanced options for target selection. CHOPCHOP uses efficient sequence alignment algorithms to minimize search times and rigorously predicts off-target binding of single-guide RNAs (sgRNAs) and TALENs. The tool generates an interactive visualization of the gene with candidate target sites, color-coded according to quality scores, and visualizes restriction sites and primer candidates for streamlined mutant generation and validation. CHOPCHOP's ease of use and speed make it a valuable resource for genome engineering.