All organisms on Earth, except for viruses, consist of cells. Yeast, for example, has one cell, while humans have trillions of cells. All cells have a nucleus, and inside nucleus there is DNA, which encodes the “program” for making future organisms. DNA has coding and non-coding segments, and coding segments, called “genes”, specify the structure of proteins, which are large molecules, like hemoglobin, that do the essential work in every organism. Practically all cells in the same organism have the same genes, but these genes can be expressed differently at different times and under different conditions. Genes make proteins in two steps. First, DNA is transcribed into messenger RNA or mRNA, which in turn is translated into proteins. The different patterns of gene expression following carefully tuned biological programs, according to tissue type, developmental stage, environment and genetic background account for the huge variety of different cells states and types. Virtually all major differences in cell state or type are correlated with changes in the mRNA levels of many genes.