In contrast to endocrine growth disorders, the hallmark of the skeletal dysplasias ("-plasia" means "growth") is disproportionate short stature. In other words, the limbs are disproportionately shorter than the trunk or vice versa. These disorders result from mutations of genes whose products are required for normal skeletal development. In most cases they are involved in endochondral ossification, the process by which the skeleton grows through the production of the cartilage template that is converted into bone. The mutated genes encode cartilage and bone extracellular matrix proteins, growth factors, growth factor receptors, intracellular signaling molecules, transcription factors, and other molecules whose functions are needed for bone growth.