Schematic diagram of CD4/PIR-A3 chimeric receptors. The four Ig-like extracellular domains of human CD4 were linked to the transmembrane and cytoplasmic domains of murine PIR-A3. The transmembrane region of PIR-A3 contains a positively charged arginine at position 632. The gray and black areas represent human CD4 and murine PIR-A3, respectively.