The proteome is the entire set of proteins that is or can be expressed by a genome, cell, tissue or organism at a certain time. It is the set of expressed proteins in a given type of cell or organism at a given time under defined conditions. Proteomics is the study of the proteome. The term has been applied to several different types of biological systems. A cellular proteome is the collection of proteins found in a particular cell type under a particular set of environmental conditions such as exposure to hormone stimulation.