Based on the suggestion from ChemBalancer -a reaction is imbalanced with either LHS or RHS species insufficient. Assuming only one molecule is missing in the reaction SMILES, an extra missing molecule symbol "@@@" of a missing molecule is added to the corresponding side of the reaction SMILES. For the paracetamol synthesis discussed above, ChemBalancer suggested RHS species insufficient for the reaction, and therefore, "@@@" symbol was added to the end of the reaction SMILES RHS, split by splitting molecule symbol "." from the original reaction SMILES, shown as "Nc1ccc(O)cc1.CC(=O)OC(C)=O>>CC(=O)Nc1ccc(O)cc1.@@@". This example is shown in Figure . ChemMLM was used to predict the masked tokens and concatenate them into a molecule. It is noticed that in some cases, more than one molecule was concatenated from the masked tokens, since the model can predict crossing-molecule token, which splits the concatenated reaction SMILES tokens into multiple molecules.