*2.1. Identification of Articles*

This systematic review was conducted according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) statement [9] and was accepted to the Prospero platform (registration CRD42019156614). PubMed and Scopus databases were searched in November 2019 using the following search strategy: ["regression"] and ["Down\* syndrome"]. Publication date and article language were not restricted; however, a filter was applied restricting articles to those involving human participants only. PubMed search fields included title and abstract, and in Scopus, title, abstract and keywords were included. As this systematic review did not contain independent research, ethical approval was not required, and consent procedures were not applicable.
