Fig. 5. PDZ domains co-occur more frequently than comparable signaling domains in the human genome. Distributions of multiple signaling domains in a single ORF as predicted by SMART (Schultz et al., 2000) are shown for SH2, SH3 and PDZ domains. ORFs are separated according to those that contain 1 (gray), 2 (purple), 3-5 (blue) and >5 (red) of a single type of domain in the same polypeptide. PDZ domains are much more likely to occur in multiple copies.