This study investigates the genetic factors influencing bone mass in adults, focusing on the lumbar spine, proximal femur, and distal forearm. The researchers measured bone mineral density (BMD) and bone mineral content (BMC) using dual photon absorptiometry in 38 monozygotic (MZ) and 27 dizygotic (DZ) twin pairs. The results show that BMD is significantly more correlated in MZ twins than in DZ twins for the spine and proximal femur, and in premenopausal twin pairs for the forearm, indicating a significant genetic contribution to bone mass at these sites. However, the genetic contribution to bone mass in the proximal femur and distal forearm is less pronounced, suggesting that environmental factors play a more significant role in these regions. The study also suggests that a single gene or set of genes may determine bone mass at all measured sites. These findings confirm the previously reported genetic contribution to bone mass in the upper limb and highlight the importance of environmental factors in the aetiology of osteopenia in the hip and wrist.This study investigates the genetic factors influencing bone mass in adults, focusing on the lumbar spine, proximal femur, and distal forearm. The researchers measured bone mineral density (BMD) and bone mineral content (BMC) using dual photon absorptiometry in 38 monozygotic (MZ) and 27 dizygotic (DZ) twin pairs. The results show that BMD is significantly more correlated in MZ twins than in DZ twins for the spine and proximal femur, and in premenopausal twin pairs for the forearm, indicating a significant genetic contribution to bone mass at these sites. However, the genetic contribution to bone mass in the proximal femur and distal forearm is less pronounced, suggesting that environmental factors play a more significant role in these regions. The study also suggests that a single gene or set of genes may determine bone mass at all measured sites. These findings confirm the previously reported genetic contribution to bone mass in the upper limb and highlight the importance of environmental factors in the aetiology of osteopenia in the hip and wrist.