A method of performing Hybridization Chain Reaction (HCR), the method comprising: providing: a first hairpin monomer and a second hairpin monomer, wherein the first hairpin monomer is labeled with a reporter molecule and/or the second hairpin monomer is labeled with a reporter molecule, a fractional initiator probe pair comprising: a first fractional initiator probe comprising a first fractional initiator comprising a sequence capable of binding to the first hairpin monomer, a second fractional initiator probe comprising a second fractional initiator comprising a sequence capable of binding to the first hairpin monomer adjacent to the region where the first fractional initiator binds, and a sample containing a target molecule; incubating the first fractional initiator probe, the second fractional initiator probe, and the target molecule; binding the first fractional initiator probe to the target molecule, binding the second fractional initiator probe to the target molecule adjacent to the region where the first fractional initiator probe binds to the target molecule so as to colocalize the first and second fractional initiators to form a full HCR initiator; binding the full HCR initiator to the first hairpin monomer which in turn binds a second hairpin monomer; triggering the growth of an HCR amplification polymer from the first and second hairpin monomers; and detecting a signal by directly or indirectly observing the reporter molecules on the first and/or second hairpin monomers incorporated within the HCR amplification polymer. a first hairpin monomer and a second hairpin monomer, wherein the first hairpin monomer is labeled with a reporter molecule and/or the second hairpin monomer is labeled with a reporter molecule, a fractional initiator probe pair comprising: a first fractional initiator probe comprising a first fractional initiator comprising a sequence capable of binding to the first hairpin monomer, a second fractional initiator probe comprising a second fractional initiator comprising a sequence capable of binding to the first hairpin monomer adjacent to the region where the first fractional initiator binds, and a first fractional initiator probe comprising a first fractional initiator comprising a sequence capable of binding to the first hairpin monomer, a second fractional initiator probe comprising a second fractional initiator comprising a sequence capable of binding to the first hairpin monomer adjacent to the region where the first fractional initiator binds, and a sample containing a target molecule; incubating the first fractional initiator probe, the second fractional initiator probe, and the target molecule; binding the first fractional initiator probe to the target molecule, binding the second fractional initiator probe to the target molecule adjacent to the region where the first fractional initiator probe binds to the target molecule so as to colocalize the first and second fractional initiators to form a full HCR initiator; binding the full HCR initiator to the first hairpin monomer which in turn binds a second hairpin monomer; triggering the growth of an HCR amplification polymer from the first and second hairpin monomers; and detecting a signal by directly or indirectly observing the reporter molecules on the first and/or second hairpin monomers incorporated within the HCR amplification polymer. <EOS>