Fluorescently labeled immunoglobulin single variable domai ns

The current invention relates to a conjugate comprising an immunoglobulin single variable domain conjugated to one or more detectable labels, wherein at least one of said labels comprises a fluorescent moiety, said fluorescent moiety having a structure chosen from formula (I) or formula (II). The current invention also relates to a conjugate comprising an immunoglobulin single variable domain conjugated to one or more detectable labels, wherein said conjugate exhibits less than 10 percent serum protein binding as measured by HPLC. The invention further relates to a pharmaceutical composition comprising aforementioned conjugate and the use of aforementioned conjugate or composition.