Motivation

As we have seen, genes affect the human body by being expressed, i.e., transcribed into mRNA and translated into proteins that react with other molecules. It is therefore highly interesting to analyze the expression profile of genes, i.e., in which tissues and at what stages of development they are expressed. From this information we can sometimes guess what the functions of these genes are. This is especially true if we discover that the expression profile of an unknown gene is similar to that of a known gene - usually, in such cases the functions of these genes are related. Another important piece of information is the level of expression of each gene. As we have seen before, different genes have different levels of expression - some are translated into proteins more often than others. With this in mind, we can know state our goal: to find which genes are expressed in each tissue, and in what level. This is easier said than done. An average tissue contains more than 10,000 expressed genes, and their expression levels can vary by a factor of 10,000. Therefore, in order to be sure we find all the genes in a tissue, we should extract more than 10⁵ transcripts per tissue. Keeping in mind that there are about 100 different types of tissue in the body, and that we are interested in comparing different growth stages (or disease stages), we can conclude that we should analyze more than 10⁷ transcripts. Obviously, we need cheap, efficient and large scale methods.