The most complex animals are the vertebrates, including humans and other mammals. The human genome is approximately 3 billion base pairs—about 20 to 30 times larger than the genomes of C. elegans, Drosophila, or Arabidopsis—and contains about 20,000 protein-coding genes. Moreover, the human body is composed of more than 200 different kinds of specialized cell types. This complexity makes the vertebrates difficult to study from the standpoint of cell and molecular biology, but much of the interest in biological sciences nonetheless stems from the desire to understand the human organism. Moreover, an understanding of many questions of immediate practical importance (e.g., in medicine) must be based directly on studies of human (or closely related) cell types.