In neuroscience, progress in understanding brain function, pathologies and neuroanatomy are often interdependent. Forebrain and cortical brain regions are mostly defined, however, midbrain to hindbrain regions are not as well characterized. Earlier this century, we discovered a structure at the transition between the midbrain to hindbrain regions, in rats, that we named the tail of the ventral tegmental area (tTVA), also known as the rostromedial tegmental nucleus (RMTg). The tVTA is now thought to be the primary inhibitory center of midbrain dopaminergic systems, inhibiting dopamine neurons of the ventral tegmental area and substantia nigra pars compacta. These structures are key in a variety of psychiatric and neurological disorders such as substance abuse, mood disorders and Parkinson's disease. Although scientific interest in the tVTA has strongly increased over the past 15 years, its localization is still unclear in mice, limiting research and its specificity. The tVTA has been characterized in rats using psychostimulant injections, which induces Fos family proteins. However, this method does not extend to mice. Here, we combined cellular (RNA in situ hybridization and immunohistochemistry) and anatomical (viral tracing) approaches to identify Sox14, a transcription factor needed for the lineage of GABAergic neurons, as a specific tVTA marker to define the boundaries and connectome of the tVTA in rats and mice brains. Using optogenetic and fiber photometry approaches in Sox14-Cre mice, we started to explore the functions of the tVTA, in particular, we began validating an aversive behavioral effect of tVTA stimulation.