Each gene generates multiple isoform variants with unique N-terminal (Nt) regions encoded by distinct specific exons (in red). PDE4 isoforms are classified upon their regulatory regions UCR1 (dots pattern) and UCR2 (line pattern). All isoforms within a specific PDE4 sub-family have identical C-terminal (Ct) regions, except the inactive PDE4A7 that contains a unique 14-residue Ct end.