All of the different molecular forms in which the protein product of a single gene can be found, including changes due to genetic variations, alternatively spliced ribonucleic acid (RNA) transcripts, and post-translational modifications.
Source:
PAC, 2018, 90, 1121. 'Terminology of bioanalytical methods (IUPAC Recommendations 2018)' on page 1162 (https://doi.org/10.1515/pac-2016-1120)