TRM cells can be detected in mice and humans. In both species, TRM cells express CD69 and CXCR3. However, in humans, one population of TRM cells expresses CD103+, while CD103 surface expression is not found in mice TRM cells. On the other hand, mice TRM cells express CD44, which mediates their liver retention and ICAM-1/LFA-1 responsible for TRM cell migration within the liver. The frequency of TRM cells within the resident T cell compartment varies between mice and humans. Some studies have described that mice TRM cells express the transcription factors Tbet, Tbx21 and KLF2, but the transcription factors upregulated by human TRM cells are not defined yet.