What is the cusp of Carabelli and what tooth it located on?

The cusp of Carabelli is a small extra cusp on the lingual side of the maxillary molars, most frequently on the mesiopalatine cusp of the maxillary first molar, but can be present on the second and third molars as well. It is said that the greater the expression of the cusp of Carabelli the greater the Neanderthal ancestry in a given individual.