CD4 and colorectal cancer: The original study by Kim et al., who discovered the colon homing function of GPR15 in mouse Tregs, observed an increased amount of Gpr15 mRNA in the human CD25−CD4+ T cell population than in Treg-enriched CD25+CD4+ T cells from colon tissues of patients with colorectal cancer (6), implying a difference between mice and humans.