An asterisk in front of a CAS number denotes that it is invalid according to the CAS registry specifications (https://www.cas.org/content/chemical-substances/checkdig). For some databases, these invalid CAS numbers are the original ones as provided by the donator with other ID information like SMILES, Chemical Name, etc. Since we are not allowed to change/correct the original chemical IDs, the system recognizes them as unique and uses them for the identification of the corresponding chemicals.