Table 4. Enrichment of framework mutations.

Full length fibronectin sequences from first and second generation naïve and binder populations were analyzed. Four framework positions demonstrated enrichment for non-wild type residues. Prevalence of these amino acids in natural homologs is shown in the two right most columns. Bottom row indicates median values of wild-type and any single mutant across all sites.

	Frequency in this work				Natural Frequency
Mutation	First Generation		Second Generation		Wild-type	Mutant
	Initial	Binders	Initial	Binders
I20V	0.5%	8%	0.5%	2%	16%	30%
S43F	0.6%	10%	1%	12%	19%	1%
P44S	17%	27%	11%	34%	23%	7%
I88T	0.1%	1%	0.2%	6%	13%	3%
median (all sites)	<0.01%	<0.01%	<0.01%	0.01%	21%	2%