A method of forming a library of fibronectin Type 3 domain polypeptides useful in screening for the presence of one or more polypeptides having a selected binding or enzymatic activity, wherein said polypeptides have the wildtype amino acid sequences in beta-strand regions A, AB, B, C, CD, D, E, EF, F, and G of the 10th fibronectin Type III module of human fibronectin which has the sequence of SEQ ID NO: 1, the method comprising the steps of: (i) aligning AB, CD, and EF amino acid loop sequences in a collection of native fibronectin Type 3 domain polypeptides, (ii) segregating the aligned loop sequences according to loop length, (iii) for a selected loop and loop length from step (ii), performing positional amino acid frequency analysis to determine the frequencies of amino acids at each loop position, wherein the AB loop length is set at 3 amino acids (AB/3), (iv) for each loop and loop length analyzed in step (iii), identifying at each position a conserved or selected semi-conserved consensus amino acid and other natural-variant amino acids, (v) for at least the AB loop, forming: a library of mutagenesis sequences expressed by a library of coding sequences that encode, at each loop position, the consensus amino acid, and if the consensus amino acid has an occurrence frequency equal to or less than a selected threshold frequency of at least 50%, a single common target amino acid and any co-produced amino acids, or a library of natural-variant combinatorial sequences expressed by a library of coding sequences that encode at each loop position, a consensus amino acid and, if the consensus amino acid has a frequency of occurrence equal to or less than a selected threshold frequency of at least 50%, other natural variant amino acids, including semi-conserved amino acids and variable amino acids whose occurrence rate is above a selected minimum threshold occurrence at that position, or their chemical equivalents, (vi) incorporating the library of coding sequences into framework Fn3 coding sequences to form an Fn3 expression library, and (vii) expressing the Fn3 polypeptides of the expression library. (i) aligning AB, CD, and EF amino acid loop sequences in a collection of native fibronectin Type 3 domain polypeptides, (ii) segregating the aligned loop sequences according to loop length, (iii) for a selected loop and loop length from step (ii), performing positional amino acid frequency analysis to determine the frequencies of amino acids at each loop position, wherein the AB loop length is set at 3 amino acids (AB/3), (iv) for each loop and loop length analyzed in step (iii), identifying at each position a conserved or selected semi-conserved consensus amino acid and other natural-variant amino acids, (v) for at least the AB loop, forming: a library of mutagenesis sequences expressed by a library of coding sequences that encode, at each loop position, the consensus amino acid, and if the consensus amino acid has an occurrence frequency equal to or less than a selected threshold frequency of at least 50%, a single common target amino acid and any co-produced amino acids, or a library of natural-variant combinatorial sequences expressed by a library of coding sequences that encode at each loop position, a consensus amino acid and, if the consensus amino acid has a frequency of occurrence equal to or less than a selected threshold frequency of at least 50%, other natural variant amino acids, including semi-conserved amino acids and variable amino acids whose occurrence rate is above a selected minimum threshold occurrence at that position, or their chemical equivalents, a library of mutagenesis sequences expressed by a library of coding sequences that encode, at each loop position, the consensus amino acid, and if the consensus amino acid has an occurrence frequency equal to or less than a selected threshold frequency of at least 50%, a single common target amino acid and any co-produced amino acids, or a library of natural-variant combinatorial sequences expressed by a library of coding sequences that encode at each loop position, a consensus amino acid and, if the consensus amino acid has a frequency of occurrence equal to or less than a selected threshold frequency of at least 50%, other natural variant amino acids, including semi-conserved amino acids and variable amino acids whose occurrence rate is above a selected minimum threshold occurrence at that position, or their chemical equivalents, (vi) incorporating the library of coding sequences into framework Fn3 coding sequences to form an Fn3 expression library, and (vii) expressing the Fn3 polypeptides of the expression library.