PSEN1 and Huntington disease: Most striking in this analysis are the findings at the embryonic stem cell and neuronal differentiation stages of HPRT-deficient cells of dysregulated signaling pathways also aberrantly expressed in Alzheimer's and Huntington's disease, including the presenilin-1 pathway, lending support to a possible functional significance of our previous findings in HPRT-deficient fibroblasts [6] and supporting the concept that LND, Alzheimer's and Huntington's diseases may share some common pathogenic mechanisms.