ChEMBL data collection and preprocessing. 2,372,647 molecules in the form of SMILES strings were collected from the ChEMBL 32 database (v. 33). Salts, and corresponding charges were removed, stereochemistry information was eliminated, and SMILES strings were sanitized. Duplicates were removed, and SMILES strings that contained atoms different than a predefined set (corresponding to the tokens 'C', 'O', 'N', 'S', 'P', 'F', 'Cl', 'Br', 'I', 'c', 'n', 'o', 's') were eliminated. Canonical SMILES strings shorter than six, and longer than 150 tokens were eliminated. Lastly, a randomized SMILES string was created for each molecule.