Illustration of HTR4 isoforms and their expression pattern in the human GI tract. (A) HTR4 isoforms HTR4a, HTR4b, HTR4d, HTR4g, HTR4i and HTR4c, without a specified 3′UTR. The common region (dark grey) encompasses exons 2-6 and is shared by all six HTR4 isoforms. The isoform-specific 3′UTRs are highlighted in red and encode unique 5-HT4R C-termini. Not drawn to scale. (A′) 5-HT4 receptor isoform transmembrane topologies indicating individual C-terminal ends predicted by Protter for visualization of proteoforms (http://wlab.ethz.ch/protter). (B) Expression pattern of HTR4 isoforms in different human GI regions. ARF1 served as mRNA integrity and loading control. Respective PCR images were cropped for figure implementation.