In addition, there are characteristic genes in the gibbon genome which had gone through a positive selection and are suggested to give rise to specific anatomical features for gibbons to adapt to their new environment. One of them is TBX5, which is a gene that is required for the development of the front extremities or forelimbs such as long arms. The other is COL1A1, which is responsible for the development of collagen, a protein that is directly involved with the forming of connective tissues as well as bone and cartilage development. This gene is thought to have a role in gibbons having stronger muscles.