Mycoplasma genitalium is a small pathogenic bacterium that lives on the ciliated epithelial cells of the urinary and genital tracts in humans. Its existence was first reported in 1981, and was eventually identified as a new species of Mycoplasma in 1983. It can cause significant morbidity in men and women, and is a co-factor in HIV transmission. It is still controversial whether or not this bacterium is to be recognized as a sexually transmitted pathogen. Specifically, it causes urethritis in both men and women, and also cervicitis and pelvic inflammation in women. Its complete genome sequence was published in 1995 (size 0.58 Mbp, with 475 genes). It was regarded as a cellular unit with the smallest genome size (in Mbp) until 2003 when a new species of Archaea, namely Nanoarchaeum equitans