EARLY-ONSET familial Alzheimer's disease (early-onset FAD) has been linked with mutations in the presenilin gene, PS1. Mutations in PS1 may affect the processing/ trafficking of β-amyloid precursor-protein (βAPP) and favour the production of toxic amyloid β-fragments that are associated with neural degeneration. This study reports the expression of a PS1-like cDNA in the carp (Cyprinus carpio) retina (the encoded protein shows 76% identity to the human PS1 protein). Carp PS1 mRNA was localized byin situhybridization to the photoreceptor cell, inner nuclear and ganglion cell layers. Expression of the PS1 gene in the rat retina was also confirmed. The retinal expression of PS1 raises the possibility that PS1 mutations also lead to neural degeneration in the retina.