Analysis of Experimental Data
You wish to explore the expression pattern of the GOI gene  in various mice tissues. (GOI - Gene of Interest - is a fictional gene, used as an example in this tutorial.)
To that end you extract mRNA from different tissues and run two equal gels (as previously described) taking special care to load equal amounts of the extracted mRNA in each well of the two gels.
Why is it important to load equal amounts of mRNA?
You then blot each gel to a separate nitrocellulose membrane. You incubate the first blot with a probe for GAPDH (a housekeeping gene) and the second blot with a probe for GOI.
 
Let us examine the expression pattern detected in the two blots.
First, let us focus on GAPDH results (lower panel of the figure to the right).
Is there a difference in GAPDH expression between the three tissues compared? 

1. No. The expression pattern and the quantities of expressed mRNA are similar in the three tissues examined.

2. Yes. No mRNA is detected in the liver.

3. Yes. Total expression of GAPDH in the brain is slightly reduced, as compared to its expression in the heart and liver, but the expression pattern is the same.

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
  
Now, let us focus on the GOI results (upper panel of the figure to the right).

Is there a difference in GOI expression between the three tissues compared? 

1. No. The expression pattern and the quantities of expressed mRNA are similar in the three tissues examined.

2. Yes. No mRNA is detected in the liver.

3. Yes. Two alternatively spliced forms of GOI are found in the heart, whereas only one appears in the brain. The relative abundance of these forms is different in these two tissues. In addition, no mRNA is detected in the liver.

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One of the uses of Northern blots is to compare mRNA quantities in different samples. Loading samples containing unequal amounts of mRNA will bias the results and will lead to wrong interpretations of the data.

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