In situ hybridization is a laboratory technique in which a single-stranded DNA or RNA sequence called a probe is allowed to form complementary base pairs with DNA or RNA present in a tissue or chromosome sample. The probe has a chemical or radioactive label attached to it so that its binding can be observed.
In situ hybridization. "In situ" is a Latin term for "in place", and then it's used in this context for detecting either RNA or DNA in the situation of the actual animal or the cells. It's a laboratory technique where it uses a probe, and this probe is usually made with DNA or RNA. It's single-stranded and it has some sort of moiety that you can detect either chemically or radioactively, and then that single-stranded is hybridized, and that's where the hybridization part comes in. You mix this probe in with your tissue sample, and you look for the single-stranded to bind in situ to the expressed mRNA or the DNA that you're looking for. You wash away the unbound probe, and then you look for where that gene is being expressed or where that piece of DNA is in the cell. This is a way of detecting things in sort of three-dimensional space, which is often very important for scientists to know where things are expressed both in terms of time and space.