Real-time quantitative reverse transcription polymerase chain reaction (qRT-PCR) is a powerful technique used to quantify gene expression. It is commonly used in research to study gene expression changes under different experimental conditions or in different tissues. One of the critical components of qRT-PCR is the use of probes that bind to the specific target RNA or DNA sequences and allow the detection and quantification of the PCR product in real-time.

There are several types of qRT-PCR probes commonly used in research. Some of the most commonly used types include:

1. TaqMan probes: TaqMan probes are hydrolysis probes that are designed to anneal to the target RNA or DNA sequence within the PCR product. The TaqMan probe contains a fluorescent reporter dye at the 5′ end and a quencher at the 3′ end. During PCR, the Taq polymerase cleaves the probe between the reporter and quencher dyes, separating them and allowing the reporter to fluoresce. The fluorescence intensity is proportional to the amount of PCR product, allowing for quantification of gene expression levels.
2. SYBR Green probes: SYBR Green is a fluorescent dye that binds to double-stranded DNA. During qRT-PCR, SYBR Green is incorporated into the PCR product, and the fluorescence intensity is proportional to the amount of PCR product. SYBR Green probes are easy to design and can be used to quantify multiple targets in a single reaction, but they are prone to non-specific amplification.
3. Molecular beacons: Molecular beacons are hairpin-shaped probes that are designed to bind to the target RNA or DNA sequence within the PCR product. The probe contains a fluorescent reporter dye and a quencher dye that are held in close proximity when the probe is in the hairpin conformation. When the probe binds to the target sequence, the hairpin opens, separating the reporter and quencher dyes, and allowing the reporter to fluoresce.
4. Scorpion probes: Scorpion probes are similar to TaqMan probes but have a stem-loop structure that increases specificity. The probe contains a fluorescent reporter dye and a quencher dye that are separated when the probe binds to the target RNA or DNA sequence within the PCR product.

The choice of qRT-PCR probe depends on several factors, such as the target sequence, the desired sensitivity and specificity, and the available instrumentation. It is important to carefully design and validate qRT-PCR probes to ensure accurate quantification of gene expression.