Because the HAM/TSP PBMCs are known to proliferate spontaneously without any mitogenic stimuli [35], PBMCs, PBMCs lacking CD4+CD25+CCR4+ T cells, and PBMCs lacking CD4+CD25−CCR4− T cells were prepared using FACS sorting from 4 HDs and 4 HAM/TSP patients and cultured for 7 days, after which the magnitude of proliferation between the 2 groups was compared. Here, CD4 is linked to tropical spastic paraparesis.