Pre-Stain/Casting

What is pre-stain/casting in gel electrophoresis?

Pre-staining is the practice of incorporating a fluorescent DNA-intercalating dye into molten agarose before it is poured into the gel tray and allowed to solidify, so that when DNA migrates through the gel under the electric field, bands become fluorescent as the DNA molecules bind dye molecules pre-distributed throughout the matrix. This approach allows bands to be monitored by UV or blue-light visualization during the run and eliminates the separate post-run soaking step that post-staining requires.

What you will find:

• Azura SmartStain Pre-Stain for visualization of DNA during gel casting

• SmartGlow™ Safe Green Pre-Stain for DNA/RNA detection in gels

• BlinkBlue® SDS-PAGE Staining Buffer for gel analysis of proteins

• SafeView Classic gel stain for day-to-day nucleic acids visualization during electrophoresis

How to choose and use pre-cast gel stains

Add ethidium bromide at the standard concentration when safety constraints allow

Ethidium bromide, the classic pre-cast stain, is typically added to cooled, molten agarose at approximately 0.5 µg/ml before pouring, providing excellent sensitivity with standard UV transilluminator imaging. Its mutagenicity requires standard handling precautions and specific waste disposal practices.

Substitute GelRed or SYBR Safe for a safer pre-cast stain option

GelRed and SYBR Safe can both be incorporated into molten agarose before casting as direct ethidium bromide substitutes at the manufacturer's recommended working concentrations, using standard UV (GelRed) or UV/blue light (SYBR Safe) for visualization on the same equipment.

Be aware that GelRed precast gels can affect DNA migration in some samples

GelRed's larger molecular size means it can cause abnormal band migration in precast gels for some samples, notably restriction digest products, compared with ethidium bromide precast gels, and this is why GelRed is specifically recommended for post-staining rather than pre-casting when migration accuracy matters most.

Cool agarose to approximately 55–60°C before adding heat-sensitive staining reagents

Some intercalating dyes degrade or lose activity at the temperatures used to keep agarose liquid for pouring, and adding stain only after the agarose has cooled to approximately 55–60°C, just before pouring, ensures consistent staining without heat-induced dye degradation.

Use blue-light gel imaging for downstream DNA recovery from pre-stained gels

If DNA will be excised from a pre-stained gel for downstream use, UV light should be minimized during excision because UV irradiation can introduce pyrimidine dimers and other photodamage that reduces downstream enzymatic efficiency. GelRed and SYBR Safe pre-cast gels can be imaged with a blue or amber light system to avoid this damage.

Specifications context

When ethidium bromide is used as a pre-cast stain, the positively charged EtBr molecules migrate toward the negative electrode during electrophoresis, moving out of the gel over time into the running buffer, which means the gel is effectively self-destaining during the run and background is reduced without a separate destaining step. As of 2026, EtBr-free labs most commonly use GelRed in post-staining applications for its superior sensitivity, while SYBR Safe is widely used as a pre-cast stain in labs that prioritize downstream DNA compatibility and blue-light gel excision.

Contact the expert team at MBP to get the right pre-staining/casting solutions for your lab.