A library of synthetic polynucleotides comprising polynucleotides that encode at least about 10 6 unique antibody binding regions whose sequences are diversified at their CDRH3, wherein the CDRH3 amino acid sequences comprise (i) an N1 amino acid sequence selected from the group consisting of G, P, R, A, S, L, T, V, GG, GP, GR, GA, GS, GL, GT, GV, PG, RG, AG, SG, LG, TG, VG, PP, PR, PA, PS, PL, PT, PV, RP, AP, SP, LP, TP, VP, GGG, GPG, GRG, GAG, GSG, GLG, GTG, GVG, PGG, RGG, AGG, SGG, LGG, TGG, VGG, GGP, GGR, GGA, GGS, GGL, GGT, GGV, D, E, F, H, I, K, M, Q, W, Y, AR, AS, AT, AY, DL, DT, EA, EK, FH, FS, HL, HW, IS, KV, LD, LE, LR, LS, LT, NR, NT, QE, QL, QT, RA, RD, RE, RF, RH, RL, RR, RS, RV, SA, SD, SE, SF, SI, SK, SL, SQ, SR, SS, ST, SV, TA, TR, TS, TT, TW, VD, VS, WS, YS, AAE, AYH, DTL, EKR, ISR, NTP, PKS, PRP, PTA, PTQ, REL, RPL, SAA, SAL, SGL, SSE, TGL, WGT; (ii) a human CDRH3 DH amino acid sequence of 0-10 amino acids in length, N- and C-terminal truncations thereof; (iii) an N2 amino acid sequence selected from the group consisting of G, P, R, A, S, L, T, V, GG, GP, GR, GA, GS, GL, GT, GV, PG, RG, AG, SG, LG, TG, VG, PP, PR, PA, PS, PL, PT, PV, RP, AP, SP, LP, TP, VP, GGG, GPG, GRG, GAG, GSG, GLG, GTG, GVG, PGG, RGG, AGG, SGG, LGG, TGG, VGG, GGP, GGR, GGA, GGS, GGL, GGT, GGV, D, E, F, H, I, K, M, Q, W, Y, AR, AS, AT, AY, DL, DT, EA, EK, FH, FS, HL, HW, IS, KV, LD, LE, LR, LS, LT, NR, NT, QE, QL, QT, RA, RD, RE, RF, RH, RL, RR, RS, RV, SA, SD, SE, SF, SI, SK, SL, SQ, SR, SS, ST, SV, TA, TR, TS, TT, TW, VD, VS, WS, YS, AAE, AYH, DTL, EKR, ISR, NTP, PKS, PRP, PTA, PTQ, REL, RPL, SAA, SAL, SGL, SSE, TGL, WGT; and (iv) a human CDRH3 H3-JH amino acid sequence, N-terminal truncations thereof. (i) an N1 amino acid sequence selected from the group consisting of G, P, R, A, S, L, T, V, GG, GP, GR, GA, GS, GL, GT, GV, PG, RG, AG, SG, LG, TG, VG, PP, PR, PA, PS, PL, PT, PV, RP, AP, SP, LP, TP, VP, GGG, GPG, GRG, GAG, GSG, GLG, GTG, GVG, PGG, RGG, AGG, SGG, LGG, TGG, VGG, GGP, GGR, GGA, GGS, GGL, GGT, GGV, D, E, F, H, I, K, M, Q, W, Y, AR, AS, AT, AY, DL, DT, EA, EK, FH, FS, HL, HW, IS, KV, LD, LE, LR, LS, LT, NR, NT, QE, QL, QT, RA, RD, RE, RF, RH, RL, RR, RS, RV, SA, SD, SE, SF, SI, SK, SL, SQ, SR, SS, ST, SV, TA, TR, TS, TT, TW, VD, VS, WS, YS, AAE, AYH, DTL, EKR, ISR, NTP, PKS, PRP, PTA, PTQ, REL, RPL, SAA, SAL, SGL, SSE, TGL, WGT; (ii) a human CDRH3 DH amino acid sequence of 0-10 amino acids in length, N- and C-terminal truncations thereof; (iii) an N2 amino acid sequence selected from the group consisting of G, P, R, A, S, L, T, V, GG, GP, GR, GA, GS, GL, GT, GV, PG, RG, AG, SG, LG, TG, VG, PP, PR, PA, PS, PL, PT, PV, RP, AP, SP, LP, TP, VP, GGG, GPG, GRG, GAG, GSG, GLG, GTG, GVG, PGG, RGG, AGG, SGG, LGG, TGG, VGG, GGP, GGR, GGA, GGS, GGL, GGT, GGV, D, E, F, H, I, K, M, Q, W, Y, AR, AS, AT, AY, DL, DT, EA, EK, FH, FS, HL, HW, IS, KV, LD, LE, LR, LS, LT, NR, NT, QE, QL, QT, RA, RD, RE, RF, RH, RL, RR, RS, RV, SA, SD, SE, SF, SI, SK, SL, SQ, SR, SS, ST, SV, TA, TR, TS, TT, TW, VD, VS, WS, YS, AAE, AYH, DTL, EKR, ISR, NTP, PKS, PRP, PTA, PTQ, REL, RPL, SAA, SAL, SGL, SSE, TGL, WGT; and (iv) a human CDRH3 H3-JH amino acid sequence, N-terminal truncations thereof.