The BEL 20 is the benchmark stock market index of Euronext Brussels. It tracks the performance of 20 most capitalized and liquid stocks traded in Belgium. In general, the index consists of a minimum of 10 and a maximum of 20 companies traded at the Brussels Stock Exchange. Since 20 June 2011, the BEL20 has contained 20 listings, with the exception a one-month period in May–June 2018 when Ablynx stock was removed following the takeover by Sanofi, only to be replaced mid June by arGEN-X. Wikipedia