Studies of twins have shown that osteoporosis may be genetically determined—up to 50%—and many genes are involved. Peak bone mass is therefore to some extent genetically programmed, and the subsequent degree of loss of bone density applies especially to trabecular (cancellous) bone. It has also been demonstrated that genes may have different effects on bones at different skeletal sites and play a part in overall bone development as well as in the degree of osteoporosis at these sites. Recently, the connection between the genes for vitamin D receptors and bone density has been of particular interest and subjects of research, though the results of such studies have been somewhat contradictory. Clinically, osteogenesis imperfecta is the most important of the hereditary osteoporoses. Other congenital syndromes with an osteoporotic component are Turner, Klinefelter, Ehlers-Danlos, Marfan and Werner. Studies of the congenital premature ageing syndrome dyskeratosis congenita have identified mutations in the genes that encode the telomerase complex. Cells of these patients have very short telomeres, and they age prematurely. The patients suffer from early greying of the hair, dental loss, osteoporosis and malignancies.