The infection of Rag2-deficient and wild type mice with Klebsiella pneumoniae, Proteus mirabilis, or a combination of both strains triggered inflammation in the presence of a complex specific-pathogen free (SPF) microbiota, but K. pneumoniae and P. mirabilis dual-association of germ-free mice did not result in the development of colitis (82). This evidence concerns the gene RAG2 and colitis.