Amino acid precursors are compounds that give amino acids after some reactions (usually hydrolysis). One of the typical amino acid precursors is aminoacetonitrile, which is converted to glycine by hydrolysis via glycine amide: NH₂CH₂CN + 2H₂O → NH₂CH₂CONH₂ + H₂O → NH₂CH₂COOH + NH₃. Hydantoins (substituted glycolylurea) are also typical amino acid precursors and have been found in carbonaceous chondrites. Complex organic polymers with large molecular weights are also possible precursors. Tholins, which are formed by reactions of mixtures of nitrogen and methane, are large complex molecules which give amino acids after hydrolysis. Amino acids are frequently detected in carbonaceous chondrites (meteorites), but the amount of amino acids recovered usually increases after hydrolysis, suggesting that some amino acids are present in the form of amino acid precursors.