Presenilin-1 (PSEN1) mutations I143T and G384A give rise to severe early onset Alzheimers's disease in two extensively studied Belgian families, AD/A and AD/B. In this study we investigated the influence of the I143T and G384A mutations on PSEN1 proteolytic processing. Hereto we analyzed PSEN1 processing in lymphoblasts by immunodetection with PSEN1-specific antibodies and densitometric analysis of the immunoreactive banding pattern. No differences were observed between presymptomatic mutation carriers, patients or escapees, demonstrating that the PSEN1 mutations I143T and G384A do not alter PSEN1 proteolytic processing in lymphoblasts.