A linker of the following formula: or a salt thereof, wherein: - X 1 and X 2 represent, independently of each other, H, a halogen atom, a (C 1 -C 6)alkoxy, or an aryloxy optionally substituted, provided that X 1 and X 2 do not represent H at the same time; - L 1 represents a group of formula L 1 '-(CO-Z') z' with L 1 ' being -(CH 2) n -, - (CH 2 CH 2 O) m -CH 2 -CH 2 -, arylene, heteroarylene, cycloalkanediyl, -(CH 2) n -arylene-, -(CH 2) n -heteroarylene-, -(CH 2) n -cycloalkanediyl-, -arylene-(CH 2) p -, -heteroarylene-(CH 2) p -, -cycloalkanediyl-(CH 2) p -, -(CH 2) n -arylene-(CH 2) p -, -(CH 2) n -heteroarylene-(CH 2) p -, -(CH 2) n -cycloalkanediyl-(CH 2) p -, -(CH 2 CH 2 O) m -CH 2 -CH 2 -arylene-(CH 2) p -, -(CH 2 CH 2 O) m -CH 2 -CH 2 -heteroarylene-(CH 2) p -, -(CH 2 CH 2 O) m -CH 2 -CH 2 -cycloalkanediyl-(CH 2) p -, -(CH 2) n -arylene-CH 2 -CH 2 -(OCH 2 CH 2) m -, -(CH 2) n -heteroarylene-CH 2 -CH 2 -(OCH 2 CH 2) m -, or -(CH 2) n -cycloalkanediyl-CH 2 -CH 2 -(OCH 2 CH 2) m -; - each W independently represents an amino acid unit; - Y is PAB-CO- z -, with PAB being the oxygen of the PAB unit being linked to CO- z; - Z is -NR 4 -(CH 2) u -NR 5 -,-NR 4 -(CH 2) u -NR 5 -CO-, -NR 4 -(CH 2) u -NR 5 -CO-(CH 2) v -, or -NR 4 -(CH 2) u -NR 5 -CO-(CH 2) v -CO-, the NR 4 group being linked to the CO group of PAB-CO; - Z' is -NR 4 -(CH 2) u -NR 5 - or -NR 4 -(CH 2) u -NR 5 -CO-(CH 2) v -, the NR 4 group being linked to the CO group of CO-Z'; - R 4 and R 5 are independently H or a (C 1 -C 6)alkyl group; - c is 0 or 1, preferably 1; - m is an integer from 1 to 15; - n is an integer from 1 to 6; - p is an integer from 1 to 6; - q is 2; - r is an integer from 1 to 24, notably from 1 to 12; - u is an integer from 1 to 6; - v is an integer from 1 to 6; - w is an integer from 0 to 5, preferably 0 or 2; - y is 0 or 1; - z is 0 or 1; - z' is 0 or 1; and - X 3 represents H when y = z = 1 and Z is -NR 4 -(CH 2) u -NR 5 - or when c = w = y = 0, z' = 1 and Z' is -NR 4 -(CH 2) u -NR 5 - and in the other cases, X 3 represents OH, NH 2 or a leaving group, wherein the leaving group is a halogen atom, a sulfonate of formula -OSO 2 -R LG, N-succinimidyloxy, 4-nitro-phenyloxy, pentafluorophenoxy or N-benzotriazoloxy, R LG representing a (C 1 -C 6)alkyl, aryl, aryl-(C 1 -C 6)alkyl or (C 1 -C 6)alkyl-aryl group, the said group being optionally substituted with one or several halogen atoms such as fluorine atoms. - X 1 and X 2 represent, independently of each other, H, a halogen atom, a (C 1 -C 6)alkoxy, or an aryloxy optionally substituted, provided that X 1 and X 2 do not represent H at the same time; - L 1 represents a group of formula L 1 '-(CO-Z') z' with L 1 ' being -(CH 2) n -, - (CH 2 CH 2 O) m -CH 2 -CH 2 -, arylene, heteroarylene, cycloalkanediyl, -(CH 2) n -arylene-, -(CH 2) n -heteroarylene-, -(CH 2) n -cycloalkanediyl-, -arylene-(CH 2) p -, -heteroarylene-(CH 2) p -, -cycloalkanediyl-(CH 2) p -, -(CH 2) n -arylene-(CH 2) p -, -(CH 2) n -heteroarylene-(CH 2) p -, -(CH 2) n -cycloalkanediyl-(CH 2) p -, -(CH 2 CH 2 O) m -CH 2 -CH 2 -arylene-(CH 2) p -, -(CH 2 CH 2 O) m -CH 2 -CH 2 -heteroarylene-(CH 2) p -, -(CH 2 CH 2 O) m -CH 2 -CH 2 -cycloalkanediyl-(CH 2) p -, -(CH 2) n -arylene-CH 2 -CH 2 -(OCH 2 CH 2) m -, -(CH 2) n -heteroarylene-CH 2 -CH 2 -(OCH 2 CH 2) m -, or -(CH 2) n -cycloalkanediyl-CH 2 -CH 2 -(OCH 2 CH 2) m -; - each W independently represents an amino acid unit; - Y is PAB-CO- z -, with PAB being the oxygen of the PAB unit being linked to CO- z; - Z is -NR 4 -(CH 2) u -NR 5 -,-NR 4 -(CH 2) u -NR 5 -CO-, -NR 4 -(CH 2) u -NR 5 -CO-(CH 2) v -, or -NR 4 -(CH 2) u -NR 5 -CO-(CH 2) v -CO-, the NR 4 group being linked to the CO group of PAB-CO; - Z' is -NR 4 -(CH 2) u -NR 5 - or -NR 4 -(CH 2) u -NR 5 -CO-(CH 2) v -, the NR 4 group being linked to the CO group of CO-Z'; - R 4 and R 5 are independently H or a (C 1 -C 6)alkyl group; - c is 0 or 1, preferably 1; - m is an integer from 1 to 15; - n is an integer from 1 to 6; - p is an integer from 1 to 6; - q is 2; - r is an integer from 1 to 24, notably from 1 to 12; - u is an integer from 1 to 6; - v is an integer from 1 to 6; - w is an integer from 0 to 5, preferably 0 or 2; - y is 0 or 1; - z is 0 or 1; - z' is 0 or 1; and - X 3 represents H when y = z = 1 and Z is -NR 4 -(CH 2) u -NR 5 - or when c = w = y = 0, z' = 1 and Z' is -NR 4 -(CH 2) u -NR 5 - and in the other cases, X 3 represents OH, NH 2 or a leaving group, wherein the leaving group is a halogen atom, a sulfonate of formula -OSO 2 -R LG, N-succinimidyloxy, 4-nitro-phenyloxy, pentafluorophenoxy or N-benzotriazoloxy, R LG representing a (C 1 -C 6)alkyl, aryl, aryl-(C 1 -C 6)alkyl or (C 1 -C 6)alkyl-aryl group, the said group being optionally substituted with one or several halogen atoms such as fluorine atoms.