Recently, several pathophysiological functions of APP have been proposed in different human diseases such as neurodevelopmental and neurodegenerative disorders including rare diseases such as autism, fragile X syndrome (FXS), amyotrophic lateral sclerosis (ALS), multiple sclerosis (MS), Lesch-Nyhan disease (LND); common and complex disorders such as AD; metabolic disorders such as diabetes; and also cancer. Here, APP is linked to Lesch-Nyhan syndrome.