In humans and some other animals, defects in the TBX5 gene expression can lead to finger-like thumbs and ventricular septal defects in which there is no separation between the left and right ventricle of the heart and are responsible for Holt-Oram syndrome.
The encoded proteins of Tbx5 and Tbx4 play a role in limb development, and play a major role in limb bud initiation specifically. For instance, in chickens Tbx4 specifies hindlimb status while Tbx5 specifies forelimb status. The activation of these proteins by Hox genes initiates signaling cascades that involve the Wnt signaling pathway and FGF signals in limb buds. Ultimately, Tbx4 and Tbx5 lead to the development of apical ectodermal ridge (AER) and zone of polarizing activity (ZPA) signaling centers in the developing limb bud, which specify the orientation growth of the developing limb. Together, Tbx5 and Tbx4 play a role in patterning the soft tissues (muscles and tendons) of the musculoskeletal system.
Genes encoding T-box proteins include:
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