Acr (anti-CRISPR) proteins are short proteins usually found in phages/prophages and other mobile genetic elements that can turn off the CRISPR-Cas system of their prokaryotic hosts. Therefore they were considered as "naturally occurring off-switch" of CRISPR-Cas and having great potential to serve as regulators/modulators of CRISPR-Cas tools for safer and more controllable genome editing.
Acr encoding genes often have an immediate downstream gene coding for a putative transcription regulator named Aca (Acr associated) protein. Published Aca proteins all have a Pfam HTH (helix-turn-helix) domain. The Acr-Aca operon is also known as Acr-Aca locus.
Please see the FAQ page of AcrFinder for more detailed intro to anti-CRISPRs.
AcrDB is a database of computationally predicted anti-CRISPR (Acr) and Acr-associated (Aca) operon database. See the Statistics page for details. Similar online databases include Anti-CRISPRdb, CRISPRminer, and AcrCatalog. Compared to these previous databases, AcrDB has the following unique features and data: