What is CERN's SIRET (France) number?

CERN is identified in the French company register with the following SIRET code (Système d'Identification du Répertoire des établissements): 394 610 661 000 16

It is useful in absence of the French VAT identification number.

Indeed, CERN (European Organization for Nuclear Research) is an Intergovernmental Organization with a diplomatic status and is not subject to any value added taxes (VAT) neither in Switzerland nor in France.
For this reason, CERN does NOT have any French VAT identification number.