= Protein isoform
A <protein isoform> is one of several related forms of a <protein>, often encoded by alternative transcripts of the same <gene>. <Alternative splicing> or alternative initiation can change the <protein sequence> and therefore remove a binding region, alter localization or change regulation. Different transcript sequences need not encode different <protein isoforms>: a change confined to an untranslated region can leave the encoded sequence unchanged. <NCBI Gene by the National Library of Medicine>[Human RAN, Gene ID 5901: transcript variants 2 and 3 encode the same protein isoform]{ref}
An experimental assignment should identify what distinguishes the forms. A shared <peptide> detected by <mass spectrometry> establishes the shared region, whereas an isoform-specific sequence supports a particular form. Intact-protein measurements can preserve combinations of sequence differences and <post-translational modifications> that are lost when all fragments are pooled. <Mapping intact protein isoforms in discovery mode using top-down proteomics by John C. Tran and others>[2011, abstract: intact-protein separation and top-down analysis]{ref}
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