ICOS and cancer: Among the genes that comprise the RNA signatures selected by these models are CXCL13, which encodes a B cell chemoattractant, CCL19, which encodes a T cell chemoattractant, ICOS, which encodes a T cell costimulator, and PDCD1, which encodes the PD-1 cell surface receptor on T cells and B cells and is a cancer immunotherapy target41.