NPY and obesity due to melanocortin 4 receptor deficiency: Likewise, phenyl butyric acid, an endoplasmic reticulum stress inhibitor as well as a histone deacetylase inhibitor, has been shown to increase Npy mRNA levels by increasing H3K9/14ac at the promoter of Npy, further implicating a potential role of histone acetylation in the development of obesity (105).