High throughput production of human proteins using different methods is being developed to make protein array approach more practical. Recently simple and efficient production of human proteins using the versatile gateway vector system has been developed [143] . In this approach, protein expression system is applied to the in vitro expression of 13364 human proteins and assessed their biological activity in two functional categories and developed "human protein factory" infrastructure which includes the resources and expression technology for in vitro proteome research. In another approach, DNA array to protein array (DAPA) is utilized, which allows the "printing" of replicate protein arrays directly from a DNA array template using cellfree protein synthesis [144] . Based on the nucleic acid programmable protein array (NAPPA) concept, high-density self-assembling protein microarray is developed to display thousands of proteins that are produced and captured in situ from immobilized cDNA templates [145] . This method will enable various experimental approaches to study protein function in high throughput. The adventage of protein-based microarrays allows the global observation of biochemical activities on an unprecedented scale, where hundreds or thousands of proteins can be simultaneously screened for protein-protein, proteinnucleic acid, and protein-small molecule interactions, as well as posttranslational modifications [146, 147] . The microarray format provides a robust and convenient platform for the simultaneous analysis of thousands of individual protein samples, facilitating the design of sophisticated and reproducible biochemical experiments under highly specific conditions [148] . The principal challenges in protein array development are 3-fold: (1) creation of a comprehensive expression clone library; (2) high-throughput protein production, including expression, isolation, and purification; (3) adaptation of DNA microarray technology to accommodate protein substrates [149] . Functional protein microarrays differ from analytical arrays in that functional protein arrays are composed of arrays containing fulllength functional proteins or protein domains (Figure 4) . These protein chips are used to study the biochemical activities of an entire proteome in a single experiment. They are used to study numerous protein interactions, such as protein-protein, protein-DNA, protein-RNA, proteinphospholipid, and protein-small molecule interactions [150, 151] . Companies have introduced protein arrays aimed not only at proteomic analysis but also functional analyses of proteins (e.g., Biacore AB, Ciphergen Biosystems Inc., Phylos Inc.). Affinity proteomics aim to produce antibodies to every protein expressed by the human genome and these will be characterized against purified antigens and tested on tissue arrays to collect information about their specificity for tissue antigens [152] . Companies are focused to produce various binding partners, for example, affibodies, monoclonal antibodies, and their fragments [153] . Protein chips will likely be the next major manifestation of the revolution in proteomics and offer another solution to analyze low abundant proteins and have the potential for high throughput applications to identify biomarkers [154] . Protein chips differ from previously described methods; whereas screening by 2DE or LC MS/MS can potentially detect any protein, and protein chips can only provide data on set of proteins selected by the investigator [155] .