Mycoplasma is a genus of small bacteria which lack cell walls. Several species are pathogenic in humans, including M. pneumoniae, which is an important cause of pneumonia and other respiratory disorders, and M. genitalium, which is involved in pelvic inflammatory diseases. An analysis of tRNA and rRNA sequences indicates that Mycoplasma belongs among the Firmicutes, and in particular is derived from the Lactobacillus-Clostridium group by losing a significant amount of DNA.