Chlamydia is the most frequently reported bacterial sexually transmitted infection in the world. Most urogenital chlamydia infections in men and women are asymptomatic, but these infections can lead to irreparable damage in the reproductive system and other tissues. Apart from the urogenital chlamydial infections, we know that chlamydia infects the gastrointestinal tract (GIT) in humans and can colonize the GIT for extended intervals without eliciting pathology. We are interested in investigating tissue tropism determinants in Chlamydia spp. because these could be targeted to development live-attenuated vaccines. Recently, we generated mutagenized isolates of the mouse pathogen Chlamydia muridarum, a close relative of the human pathogen Chlamydia trachomatis which causes chlamydia. One mutant that we isolated is significantly attenuated in murine gastrointestinal tissues compared to wild type, but retains its pathogenicity in the murine urogenital tract. Using novel genetic techniques, whole-genome sequencing, and complementation using newly developed vector systems we identified a chromosomal factor, tc0600, that we believe mediates the altered tissue tropism phenotype of this mutant in mice. Notably, the Chlamydia trachomatis ortholog of tc0600 has been linked to chlamydial GIT tropism in humans.