This paper deals mainly with AD in relation to retinoic acid receptors (RARs: RARα, β and γ) and their ligands (retinoids), such as the endogenous RAR ligand all-trans-retinoic acid (RA), taking into account knowledge about PD, ALS and other neurodegenerative diseases. This evidence concerns the gene RARS1 and neurodegenerative disease.