Indeed, just 2 years after the discovery of HAM/TSP, Usuku et al. (1988) identified human leukocyte antigen (HLA)-DR1 and -DR4 as two HLA-DR serotypes highly represented in HAM/TSP, hypothesizing that HAM/TSP and ATL correspond to two populations of HTLV-1 carriers with distinct immune responses to HTLV-1. This evidence concerns the gene TNFRSF10A and tropical spastic paraparesis.