Relative frequency of NGR in proteins involved in cell adhesion. Frequency of XGR, NXR, and RGD sequence in vertebrate proteins (SWISS-PROT database) classified with the keywords “adhesion” (n = 1010, A), “receptor” (n = 4561, B), and “enzyme” (n = 857, C), relative to the frequency in total vertebrate proteins (n = 39 525). NGR and DGR are the most frequent motives in the “adhesion” category. The analysis was performed using Search Databases for Regular Expression software (Center for Cancer Research, Massachusetts Institute of Technology, Cambridge, MA).⁸⁷  Solid and dashed horizontal lines represent means and plus or minus SD, respectively.