The MEROPS database, available at http://merops.sanger.ac.uk, is a comprehensive resource for proteolytic enzymes, their substrates, and inhibitors. It employs a hierarchical classification system to group homologous peptidases and protein inhibitors into species, families, and clans. Recent developments include a community annotation project, an Internet-based data entry form, and a new display showing the intron/exon structures of eukaryote peptidase genes. The database now supports filtering of peptidases from completely sequenced bacterial genomes and has been updated to handle the large number of homologues in some families using HMMER searches. The number of sequences in MEROPS has more than doubled since July 2011, and the database now includes over 4000 peptidases, each with a unique MEROPS identifier and detailed summaries. The community annotation project involves experts contributing summaries for peptidases, and the database has received over thirty contributions as of March 2013. Other recent developments include gene displays, organism pages, and improved literature links.The MEROPS database, available at http://merops.sanger.ac.uk, is a comprehensive resource for proteolytic enzymes, their substrates, and inhibitors. It employs a hierarchical classification system to group homologous peptidases and protein inhibitors into species, families, and clans. Recent developments include a community annotation project, an Internet-based data entry form, and a new display showing the intron/exon structures of eukaryote peptidase genes. The database now supports filtering of peptidases from completely sequenced bacterial genomes and has been updated to handle the large number of homologues in some families using HMMER searches. The number of sequences in MEROPS has more than doubled since July 2011, and the database now includes over 4000 peptidases, each with a unique MEROPS identifier and detailed summaries. The community annotation project involves experts contributing summaries for peptidases, and the database has received over thirty contributions as of March 2013. Other recent developments include gene displays, organism pages, and improved literature links.