The invention relates to a polypeptide, such as an Affimer polypeptide, comprising an amino acid sequence having at least 80% identity to amino acid residues 1 to 11, 13 to 15, 17 to 19, 21 to 25, 27to 28, 35 to 37, 39, 41, 43 to 44, 46 to 47, 49 to 50, 52 to 53, 55 to 58, 63 to 64, 66, 68 to 82, 84 to 85, and 87 to 98 of SEQ ID NO: 1; characterised in that said polypeptide comprises one or moremutations relative to SEQ ID NO: 1 selected from the group consisting of: T51L, T51V, M65V, N32G, A59I, L38A, V20I, A40I, L38V, A12I, A12V, I16L, V20L, Q26E, E29M, T31K, N32D, N32H, T34V, T34R, T34D,T34P, A40V, Q42D, T45I, T45V, V48E, V48G, V48A, T51F, T51A, A59L, L67I, (V20I, L38A), (V20L, L38A), (V20I, L38V), (V20L, L38V), (E29K, K30E, E33K), (Y54D, T83D, Q86E), (A59L, G60N, D61G, N62K), (A59V,D61N, N62K), (G60N, D61G, N62K), (G60N, delta D61, N62G), delta D61, (A59L, G60N, delta D61, N62G), (A59V, G60N, D61G, N62K), (A59I, G60N, D61G, N62K), (A59I, G60N, delta D61, N62G), (A59V, G60N, delta D61, N62G), (A59V, delta D61), (G60P, delta D61, N62P), (G60P, D61P, N62K), (G60P, delta D61, N62G), (G60P, D61G, N62K), (D61N, N62K) and (T83D, Q86E). The invention also relates to various methodsand nucleic acids.