'It's a Knockout!' provides an update of some of the latest mouse knockouts in TBASE (http://www.jax.org/tbase/ and Ref. 1). The column provides a concise phenotypic profile of novel mutants and renders their complete characterization directly accessible to Web users, via unique and unchanging accession numbers (TBASE identities). Where possible, interesting knockouts will be grouped according to gene families, application or phenotypic similarities.