Using a cut-off of >25% inhibition of proliferation, none out of eight HAM/TSP patients could be defined as an IFN-α-responder, whereas six out of eight HAM/TSP patients were high-dose AA-responders (Fisher's exact test, p = 0.009), independent of the method used (thymidine incorporation and cell counting). This evidence concerns the gene IFNA2 and tropical spastic paraparesis.