The discovery that allelic forms of Prnp controlled mouse scrapie incubation times led to studies to test whether rare familial forms of GSS were also influenced by the nucleic acid sequence of the human PrP gene, PRNP. A missense mutation at codon 102 was linked to GSS [55], and transgenic mice that expressed high levels of Prnp with the mouse equivalent of the human GSS mutation spontaneously developed an ND that could be transmitted with short incubation times to transgenic mice that expressed GSS-mutant PrP at low levels and that developed spontaneous disease only late in life [56,57]. The gene discussed is PRNP; the disease is scrapie.