Rodents are the most widely used model organism in neuroscience and biomedical research. Rats and mice differ significantly in gene expression, social behaviors, and physiology, giving them unique advantages and disadvantages as the favorable model in any particular research. For example, mice were more commonly used due to their larger “genetic toolbox,” which includes transgenic and “knockout”/”knock-in” mice. However, adult rats typically weigh up to ten times more than adult mice. Their larger size makes surgery and other invasive procedures such as catheter insertion easier to perform, while mice are chosen for optogenetics experiments because light passes more easily through their smaller brains and can therefore reach deeper regions of the brain.