For a long period of time, geneticists believed that one gene codes for one polypeptide. This hypothesis has been re-evaluated for two reasons. Firstly, some genes do not encode polypeptides, but functional RNA molecules. Secondly, due to the phenomenon of alternative splicing, some genes can encode several similar but not identical polypeptides. This phenomenon is present only in eukaryotes and it is based the fact that different parts of some genes can be used during gene expression.