Abstract: A humanised antibody capable of binding to the MUC1 mucin antigen comprises a light chain and a heavy chain. The variable region of the light chain (VL) comprising an amino acid sequence which is substantially homologous with the sequence of FIG. 1A and the variable region of the heavy chain (VH) comprising an amino acid sequence which is substantially homologous with the sequence of FIG. 1B. The amino acid residue at position 46 on VL is backmutated to arginine, and the amino acid residue at position 47 on VH is backmutated to leucine. The humanised antibody has use in the diagnosis and/or treatment of cancer.