This family consists of several Toluene-4-monooxygenase system protein B (TmoB) sequences. Pseudomonas mendocina KR1 metabolizes toluene as a carbon source. The initial step of the pathway is hydroxylation of toluene to form p-cresol by a multicomponent toluene-4-monooxygenase (T4MO) system. TmoB adopts a ubiquitin fold. Although TmoB is a component of the T4MO system, its precise role remains unclear.