A covalently reactive ligand analogue (CAL) of formula (1): wherein, L1...Lx...Lm are components defining a ligand determinant, LX is a componentunit of the ligand determinant selected from the group consisting of an aminoacid residue, sugar residue, a fatty acid residue and a nucleotide, L is afunctional group of LX, Y" is atom, covalent bond or linker, Y is an optionalcharged or neutral group Y is a covalently reactive electrophilic group thatreacts specifically with a receptor that binds to said ligand determinant, andn is an integer from 1 to 1000 m is an integer from 1 to 30.
