Pin1: An enzyme that permits brain cells to function normally. Pin1 may also protect against Alzheimer's disease. The enzyme is clearly involved in regulating mitosis (cell division). The Pin1 level in the brains of Alzheimer patients is greatly reduced as compared to age-matched normal brains. The depletion of Pin1 may induce mitotic arrest and apoptotic cell death in the brain and hence contribute to Alzheimer's disease. The Pin1 gene is on chromosome 19 in band 19p13. Pin1 is also known as poly1 isomerase or peptidyl-prolyl cis/trans isomerase.