HPRT1 and early-onset autosomal dominant Alzheimer disease: As we have confirmed by functional studies, the transcriptional characterization of human fibroblasts knocked down for HPRT expression through RNA interference has correctly identified aberrations in HPRT deficiency of a number of important signaling pathways, especially those identified in gene ontology categories as the Wnt signaling and the Alzheimer's disease/presenilin pathways that play roles in the pathogenesis of two common human neurodegenerative and neurodevelopmental disease – Parkinson's and Alzheimer's diseases.