Table 1

Measures of gene expression in ascending order of complexity 
Measure 
Algebra formulae 
Description 
Example in skeletal muscle context 
Expression 

Average (normalized) expression of the ith gene across the n samples (eg. biological replicates) of experimental condition A and where each x_{i,k} corresponds to the expression of the ith gene in the kth sample (k = 1, …, n). 
MYL2 is abundant, MSTN is intermediate 
Differential Expression 

Difference in the expression of the ith gene in the two conditions under scrutiny, A and B (eg. healthy and diseased, two breeds, two diets, two time points, …). Note that
it is not a requirement to have the same number of samples surveyed in the two conditions. 
MYL2 relatively strongly, definitely not MSTN 
CoExpression 

Similarity of expression profile (typically and shown here the Spearman correlation
coefficient) between the ith and the jth genes across the n samples of condition A. 
MYOD1 and MYOG 
Differential Co Expression 

Difference in the coexpression between the ith and the jth genes in the two conditions under scrutiny, A and B. Note that it is not a requirement to have the same number of samples surveyed in
the two conditions. 
MSTN and MYL2 
CoDifferential Expression 

Similarity of the profile of differential expression of genes i and j across the levels of another experimental design effect such as time points. Two
conditions, A and B, are being surveyed across a series of developmental time points. 
MYL2 and MYL3 
Hudson et al.
Hudson et al. BMC Genomics 2012 13:356 doi:10.1186/1471216413356