In biology, a probe is a single strand of DNA or RNA that is complementary to a nucleotide sequence of interest.
RNA probes can be designed for any gene or any sequence within a gene for visualization of mRNA, lncRNA and miRNA in tissues and cells. FISH is used by examining the cellular reproduction cycle, specifically interphase of the nuclei for any chromosomal abnormalities. FISH allows the analysis of a large series of archival cases much easier to identify the pinpointed chromosome by creating a probe with an artificial chromosomal foundation that will attract similar chromosomes. The hybridization signals for each probe when a nucleic abnormality is detected. Each probe for the detection of mRNA and lncRNA is composed of ~20-50 oligonucleotide pairs, each pair covering a space of 40–50 bp. The specifics depend on the specific FISH technique used. For miRNA detection, the probes use proprietary chemistry for specific detection of miRNA and cover the entire miRNA sequence.
Probes are often derived from fragments of DNA that were isolated, purified, and amplified for use in the Human Genome Project.  The size of the human genome is so large, compared to the length that could be sequenced directly, that it was necessary to divide the genome into fragments. (In the eventual analysis, these fragments were put into order by digesting a copy of each fragment into still smaller fragments using sequence-specific endonucleases, measuring the size of each small fragment using size-exclusion chromatography, and using that information to determine where the large fragments overlapped one another.) To preserve the fragments with their individual DNA sequences, the fragments were added into a system of continually replicating bacteria populations.  Clonal populations of bacteria, each population maintaining a single artificial chromosome, are stored in various laboratories around the world.  The artificial chromosomes (BAC) can be grown, extracted, and labeled, in any lab containing a library. Genomic libraries are often named after the institution in which they were developed. An example being the RPCI-11 library, which is named after Roswell Park Comprehensive Cancer Center (formerly known as Roswell Park Cancer Institute) in Buffalo, New York. These fragments are on the order of 100 thousand base-pairs, and are the basis for most FISH probes.