When using the ABI StepOnePlus for mouse gene expression, how should I choose between ROX and dye-based methods?

A 10 μl system can be used on the StepOnePlus, but when template input is low or Ct values are high, pipetting errors have a large impact on reproducibility. It is recommended to add 2–3 μl of cDNA to 10 μl, prepare and aliquot the primer master mix in advance, use calibrated low-retention tips, and include at least 2 technical replicates. If budget allows, a 20 μl system is more robust, especially for edge wells. Keeping costs within budget requires finding domestic bulk packaging, but do not compromise on the quality of sealing films and consumables.

Buying the wrong low ROX will not make quantification completely impossible, but it will affect normalization, especially when there are large differences in liquid volume between wells, causing Ct drift. Tissue sample Ct values are relatively high. If there is little change when cDNA is diluted 5-fold versus 10-fold, it may be due to low target gene expression or poor primer design. It is recommended to also run a reference gene (such as GAPDH/Actb) and check its Ct. If the reference gene Ct is also high, it may be a cDNA or RNA quality issue rather than the dilution factor. To assess the stability of a 10 μl system, run 8-10 replicates with the same cDNA and check whether the Ct SD is less than 0.3.

For StepOnePlus, be sure to choose the ROX-containing version; usually the low ROX compatible version is sufficient. Do not buy the high ROX version, as high ROX is intended for instruments such as the 7500/QuantStudio. If the wrong version is purchased, the baseline signal will be abnormal and ΔRn may be suppressed, but quantification can still be performed if the standard curve is properly prepared. To be safe, validate the baseline and amplification efficiency with standards on the first plate of reagent. If Ct is high, do not blame the reagent first; diluting tissue cDNA 5-10 fold is not necessarily appropriate, and a template amount gradient can be performed.

For cost per well, it is best not to calculate only the reagent; you should also include primers, PCR plates/film, and pipette tips. For more than 60 samples, if each sample has 2-3 replicates, one batch is about 200 wells; choosing domestic SYBR Green in bulk packaging can generally meet the need. The key is to first validate primer specificity: a single peak in the melting curve, no nonspecific bands on agarose gel, and amplification efficiency of 90-110%. If the primers themselves have low efficiency, switching to ROX or cheaper reagents will not help. For the low ROX version of StepOnePlus, universal low ROX reagents can be purchased.

For mouse liver mRNA expression, I generally dilute tissue cDNA 5–20 fold, but even at 10-fold dilution, high-abundance genes in the liver may still show high Ct values, suggesting possible issues with RNA quality or reverse transcription efficiency. It is recommended to first measure RNA concentration and A260/280, use equal amounts of RNA (e.g., 500 ng) for reverse transcription, and then prepare serial dilutions of the stock, 2-fold, 5-fold, and 10-fold, selecting the dilution factor that yields Ct values between 15 and 30 with good linearity. With over 60 samples, do not introduce batch-to-batch variation just to save one dilution step.