Truncated progesterone receptor (PR) isoforms. The upper part of each diagram shows the PR gene; darker shading indicates regions removed by splicing, lighter regions those retained in mRNA. The lower part of each diagram shows the predicted protein structure. PR-S: retention of intronic sequence termed exon S and initiation of transcription within exons 4 to 8. PR-T: retention of intronic sequence termed exon T and initiation of transcription within exons 4 to 8. PR-S and PR-T mRNA are likely to give rise to identical proteins that are amino-terminally truncated, lacking AF-1, AF-2 and the DNA binding domain. PR-i45: retention of two intronic sequences termed exons i45a and i45b leads to a change in reading frame and truncation of the protein E region so the PR-i45 protein lacks a functional ligand-binding domain and dimerisation domain. PR-M: inclusion of an intronic 5' untranslated region sequence causes amino-terminal truncation and leads to encoding of a protein with a 5' signal sequence and complete ligand-binding domain and dimerisation domain, consistent with a function as a membrane bound receptor.