Compound having substance that has affinity for soluble protein, cleavable moiety, and reactive group, or salt thereof

The present invention provides a technique that enables modification of a soluble protein, particularly regioselective modification of a soluble protein. More specifically, the present invention provides a compound having a substance that has affinity for a soluble protein, a cleavable moiety, and a reactive group, the compound being represented by formula (I): A-L-B-R [in the formula, A is a substance that has affinity for a soluble protein, L is a cleavable linker that is a divalent group containing a cleavable moiety, B is (a) a divalent group containing a bioorthogonal functional group or (b) a divalent group not containing a bioorthogonal functional group, and R is a group that is reactive to the soluble protein], or a salt thereof.