Cyanine dye labelling reagents

Disclosed are cyanine dyes that are useful for labeling and detecting biological and other materials. The dyes are of formula (I) in which groups R3 and R4 are attached to the Z1 ring structure and groups R5 and R6 are attached to the Z2 ring structure, and n=1, 2 or 3; Z1 and Z2 independently represent the carbon atoms necessary to complete a one ring, or two-fused ring aromatic system; at least one of groups R1, R2, R3, R4, R5, R6 and R7 is the group -E-F where E is a single bond or a spacer group and F is a target bonding group; one or more of groups R11, R12, R13 and R14 are independently selected from the group -(CH2)k-W, where W is sulphonic acid or phosphonic acid and k is an integer from 1 to 10. The dyes may be used in fluorescence labeling applications, where the presence of one and preferably multiple water solubilising groups attached to the 3-position of the indolinium ring reduces dye-dye interactions, and hence dye-dye quenching, particularly where multiple dye molecules are attached to components such as nucleic acids, oligonucleotides, proteins and antibodies.