DNA microarray
Nucleic acid hybridization assays have been used for decades to detect specific sequences of DNA or RNA, with a DNA microarray precursor used as early as 1965. In such assays, positive control oligonucleotides are necessary to provide a standard for comparison of target sequence concentration, and to check and correct for nonspecific binding; that is, incidental binding of the RNA to non-complementary DNA sequences. These controls became known as "spike-ins". With the advent of DNA microarray chips in the 1990s and the commercialization of high-throughput methods for sequencing and RNA detection assays, manufacturers of hybridization assay "kits" started to provide pre-developed spike-ins. In the case of gene expression assay microarrays or RNA sequencing (RNA-seq), RNA spike-ins are used.