To test whether removal of cathepsin D enhanced tau-induced neurodegeneration through abnormal cell-cycle activation, we immunostained brains from 10-day-old flies for proliferating cell nuclear antigen (PCNA), an S-phase cell-cycle marker abnormally upregulated in brains of Alzheimer's disease patients and tau-expressing flies. This evidence concerns the gene MAPT and early-onset autosomal dominant Alzheimer disease.