wherein the medically useful metal ion-binding domain is Y1 !n, Y1 --(R2)--Y1 !n or Y1 --(R2)--Y2 !n, in which n is a number between 1 and about 6 and Y1 and Y2 are amino acids with a sulfur, nitrogen or oxygen which is available for binding to metal ions, or can be made available for binding to metal ions; the biological-function domain is an amino acid sequence containing from 1 to about 20 amin