Despite clear evidence from mouse linkage studies of multiple genetic loci affecting incubation periods of prion disease[1;2], no specific human genes have been identified aside from the prion protein gene (PRNP). A recently characterised highly conserved gene, SPRN, encodes Shadoo (Sho, shadow of prion protein) which has protein homology and possible functional links with the prion protein. We carried out a genetic screen of the open reading frame of SPRN by direct sequencing in 522 prion disease patients, including 107 with variant Creutzfeldt-Jakob disease (vCJD), and 861 healthy controls. We identified a common coding variant of SPRN, two further single nucleotide polymorphisms (SNPs) and three rare insertion or deletion variants. A single base pair insertion at codon 46, predicted to cause a frameshift and potentially a novel protein, was found in two vCJD patients but not controls (P = 0.01). Two linked SNPs, one in intron 1 the other a missense variant at codon 7, were associated with risk of sporadic CJD (P=0.009). These data justify the functional genetic characterisation of SPRN and support the involvement of Shadoo in prion pathobiology.