Transmissible spongiform encephalopathies (TSE) are a group of invariably fatal neurodegenerative diseases and include scrapie in sheep, bovine spongiform encephalopathy (BSE) in cattle, chronic wasting disease in deer and elk, and Kuru disease, Creutzfeldt-Jakob disease (CJD) and variant CJD in humans. The pathological effects of disease occur predominantly in the CNS (central nervous system), where common hallmarks include vacuolation, gliosis, accumulation of a protease-resistant, abnormally folded isoform of the prion protein (PrPSc) and neuronal cell death. Lack of understanding of the molecular mechanisms underlying disease pathogenesis, particularly in non-CNS tissues, means that there are currently no effective strategies for early diagnosis or therapeutic intervention of TSEs. Here we report the first identification of a molecular marker that is easily detectable in readily accessible tissues. We demonstrate that a dramatic decrease in expression of a transcript specific to erythroid lineage cells is a common feature of TSEs. Our findings indicate a previously unrecognized role for involvement of the erythroid lineage in the etiology of TSE pathogenesis and should provide a new focus for research into diagnostic and therapeutic strategies.