“Classification” module provides types and subtypes of the CRISPR-Cas systems based on Cas genes contents and distinctive gene architectures according to the most recent classification studies (Makarova et al 2015 and Shmakov et al 2017) . In this module, users can quickly browse membranes of each subtype of CRISPR-Cas systems. Each item contains information, such as Refseq ID, Genebank ID and the name of the bacterial organism, and its Cas gene content. It is convenient for users to lookup the gene composition and architecture of the respective Cas operons of each bacterium.