Livak, K. J., & Schmittgen, T. D. (2001). Analysis of relative gene 
expression data using real-time quantitative PCR and the 2- DDCT method. 
methods, 25(4), 402-408.

Abstract
The two most commonly used methods to analyze data from real-time, 
quantitative PCR experiments are absolute quantification and relative 
quantification. Absolute quantification determines the input copy number, 
usually by relating the PCR signal to a standard curve. Relative quantification 
relates the PCR signal of the target transcript in a treatment group to that of 
another sample such as an untreated control. The 2- DDCT method is a convenient 
way to analyze the relative changes in gene expression from real-time 
quantitative PCR data.