text: The runaway success of DNA microarrays has generated much enthusiasm for protein microarrays. However, protein microarrays have not quite taken off as expected, even with the necessary tools and know-how from DNA microarrays being in place and ready for adaptation. One major reason is that protein microarrays are much more laborious and technically challenging to construct than DNA microarrays.
  The traditional methods of producing protein arrays require the separate in vivo expression of hundreds or thousands of proteins, followed by separate purification and immobilization of the proteins on a solid surface. Cell-free protein array technology attempts to simplify protein microarray construction by bypassing the need to express the proteins in bacteria cells and the subsequent need to purify them. It takes advantage of available cell-free protein synthesis technology which has demonstrated that protein synthesis can occur without an intact cell as long as cell extracts containing the DNA template, transcription and translation raw materials and machinery are provided. Common sources of cell extracts used in cell-free protein array technology include wheat germ, Escherichia coli, and rabbit reticulocyte. Cell extracts from other sources such as hyperthermophiles, hybridomas, Xenopus oocytes, insect, mammalian and human cells have also been used.
  The target proteins are synthesized in situ on the protein microarray, directly from the DNA template, thus skipping many of the steps in traditional protein microarray production and their accompanying technical limitations. More importantly, the expression of the proteins can be done in parallel, meaning all the proteins can be expressed together in a single reaction. This ability to multiplex protein expression is a major time-saver in the production process.