To characterize the proliferation and phenotype status of T cells undergoing HSP, primary purified CD4+ T cells were cultured in the presence of both IL-7 and IL-15 (designated here as HSP culture) or stimulated via the TCR using immobilized anti-CD3 and anti-CD28 in the presence of IL-2 (designated as TCR culture) as control. This evidence concerns the gene IL15 and hereditary spastic paraplegia.