The present invention provides a plurality of pairs of proximity probes, each pair being capable of binding to a different target analyte, wherein the first and second proximity probes of each pair of probes comprise universal oligonucleotides conjugated to their analyte binding moieties, and hybridised to the universal oligonucleotides are different tag oligonucleotides comprising universal complement domains common to all tag oligonucleotides and unique domains unique to each tag oligonucleotide, as well as methods for their production.