Synthesis and degradation of mRNA together with synthesis and degradation of corresponding protein, this four-step-expression confers great fitness to all organisms. Transcription rate and mRNA stability both are essential for circadian expression of clock genes. In many cases, transcription rates and half-lives of mRNAs and corresponding proteins are not necessarily tightly linked with each other. The methods for measuring four-step-expression should be carefully selected and the experimental conditions should be strictly controlled.