Hybridization is the base pairing of complementary strands of RNA and DNA. The complementary base pairing may be between two strands of DNA, two strands of RNA, or a strand of DNA and a strand of RNA. In most clinical molecular testing, one of the strands is synthetic and the resulting double-stranded nucleic acid is referred to as a hybrid. Often the synthetic nucleic acid strand is short and is then called an oligonucleotide. A probe is another name for a synthetic strand of nucleic acid that hybridizes; probes often have attached labels.