3' Hairpin OCPs design allows conversion of unused OCPs to inert double-stranded form. If the OCP is not circularized, the single stranded region of the unligated linear probe can bind P1 primer and be copied, a process that can lead to misamplification of unligated probe. To reduce the potential for misamplification, a 3' hairpin has been designed into the OCP. During ERCA reaction, 3' hairpin is extended by polymerase, resulting in an extended hairpin with minimal single strandedness, which is less likely to participate in faulty priming reactions that result in misamplification.
Alsmadi et al. BMC Genomics 2003 4:21 doi:10.1186/1471-2164-4-21