Red blood cell differentiation represents a convenient model for examination of molecular mechanisms in coordinate regulation of gene expression. The molecular mechanism of action of these substances is not yet fully explained, but by comparison with the similar action of some hormones it is very probable that they bind with specific receptors of cell nuclei and cause activation and expression of particular genes. Gene regulation as an aspect of cell differentiation usually is examined by two methods: one based on genomic organization during development, when a multicellular organism generates from a single cell, and step-by-step, through proliferation, new generations of specialized cells are made. The primary totipotent cells of the early embryo give rise to stem cells specific to three distinct layers; ectoderm, mesoderm, and entoderm, from which starts the development of all definitive tissues. Globin genes are expressed only in erythroid cells. Many authors agree that deoxyribonucleic acid (DNA) methylation is an important mechanism in gene regulation.