The rs8099917 and rs12979860 polymorphisms of the IL-28B gene play an important role in the pathogenesis of HAM/TSP; specifically, the TT genotype of the rs8099917 polymorphism is directly associated with an increased expression of the interleukin IL-28B, which causes an abnormal proliferation of CD4+ T lymphocytes, a mechanism that may promote chronic inflammation and increase the risk of development of mentioned diseases, which would result in the development of diseases such as ATL and myelopathy associated to HTLV-1. The gene discussed is IFNL3; the disease is tropical spastic paraparesis.