Correction to 'The STRING database in 2021: customizable protein–protein networks, and functional characterization of user-uploaded gene/measurement sets'. Issue 18 (16th September 2021) - Szklarczyk, Damian; Gable, Annika L; Nastou, Katerina C; Lyon, David; Kirsch, Rebecca; Pyysalo, Sampo; Doncheva, Nadezhda T; Legeay, Marc; Fang, Tao; Bork, Peer; Jensen, Lars J; von Mering, Christian