The recent availability of platform technologies for high throughput proteome analysis has led to the emergence of integrated messenger RNA and protein expression data. The Pearson correlation coefficients for these data range from 0.46 to 0.76. In these integrated studies, serial analyses of gene expression and DNA microarrays have been used to quantify the transcriptome, while proteome analysis has been based on two-dimensional gel electrophoresis, isotope coded affinity tags and multidimensional protein identification technology. This paper provides a comprehensive review of the analytical techniques used in these studies and explores the extent to which the choice of experimental methodology can bias the correlation or the ability to detect proteins.