One way of bringing several causal components together would be to think of HAM/TSP as a disease that occurs in people (1) with a genetic tendency (based on HLA among other genes) to develop inflammatory conditions, (2) carrying an HTLV-1 strain with strong expression of antigenic Tax protein, and (3) infected with HTLV-1 via sexual intercourse (sporadic, late onset HAM/TSP) or via breastfeeding (familial, early onset HAM/TSP). This evidence concerns the gene CNTN2 and tropical spastic paraparesis.