Gene expression comparison between different species(!) - (Jun/26/2005 )

Hi all experts,
I am a green grass to gene expression analysis. I work with qPCR to analyze gene expression. My work usually compares gene expression difference between treated and non-treated(control) groups, by calculating the ratio(Treated/non-treated). Before calculating this ratio, the values of both treated samples and controls are all normalized firstly to the house-keeping gene(HKG).
It sounds like an IQ question. Recently I get a challenging job. I have to compare the expression levels of a certain gene in different species(!). I have subjected all the samples to the HKG. It shows me the absolute expression level is different between my samples. Let's say, two samples(A and B ) have the same expression level of 100 copies of the HKG expression, C has a expression of 150 copies. I assume that this variation is due to the up-stream steps and all gene expressions are parallel to the HKG level. qPCR shows me A has 200 copies of gene-of-interest, B has 300 copies and C has also 300 copies. And A, B and C are different vertebrate sorts. Where I have to take consideration and caution of before I can say B has the highest expresson level, whereas A and C are the same?
Do I need to know the exact molecule number/ul of my samples?
Thanks a lot in advance.
_________________
Ribosoul

Knowledge makes life better

Use the DeltadeltaCt method. this method gives you an expression level normalized by HKG expression. You can compare samples from different species with different HKG expression levels.