The present invention relates to a polypeptide binding to fibroblast growth factor receptor 3 isoforms 3b and 3c (FGFR3b and FGFR3c), wherein the polypeptide comprises an amino acid sequence selected from the group consisting of: (a) GVTLFVALYDYEVYGPTPMLSFHKGEKFQIL(X1)(X2)(X3) (X4)GPYWEARSL(X5)TGETG(X6)IPSNYVAPVDSIQ (SEQ ID NO: 1), wherein amino acid positions (X1) to (X6) may be any amino acid sequence； (b) an amino acid sequence which is at least 95% identical to the amino acid sequence of (a), wherein the identity determination excludes amino acid positions (X1) to (X6) and provided that the amino acid sequence EVYGPTPM (SEQ ID NO: 2) in amino acid positions 12 to 19 of SEQ ID NO: 1 is conserved and the amino acids P and Y in amino acid positions 37 and 38 of SEQ ID NO: 1 are conserved； (c) GVTLFVALYDYEVMSTTALSFHKGEKF QILSQSPHGQYWEARSLTTGETG(X6)IPSNYVAPVDSIQ (SEQ ID NO: 19), wherein the amino acid position (X6) may be any amino acid； and (d) an amino acid sequence which is at least 95% identical to the amino acid sequence of (c), wherein the identity determination excludes amino acid position (X6) and provided that the amino acid sequences EVMSTTA (SEQ ID NO: 20) in amino acid positions 12 to 18 of SEQ ID NO: 19 and SQSPH (SEQ ID NO: 21) in amino acid positions 31 to 35 of SEQ ID NO: 19 are conserved and the amino acids Q and Yin amino acid positions 37 and 38 of SEQ ID NO: 19 are conserved.
