The process of developing from a single cell into a fully functional multicellular organism is quite arduous and can be perturbed by both genetic and environmental factors. Large chromosomal rearrangements and loss-of-function mutations in essential genes often result in early embryonic lethality. Many birth defects from mild to severe can be attributed to defects in developmental genes. Exposure to environmental factors such chemicals or pathogens can also result in developmental abnormalities. Even cancers can be attributed to misregulation of developmental genes.