Required for pectoral fin formation. Together with tbx5b, involved in eye and heart development. Required for the looping stage of heart development. May bind to the core DNA motif of promoters. Monomer. Homodimer (via the T-box); binds DNA as homodimer. Shuttles between the cytoplasm and the nucleus. Expressed in the dorsal optic cup of developing eye, pectoral fin buds and heart. At 31 hpf, when the pectoral fin buds have begun bulging outwards, restricted expression is detected throughout the mesenchyme of the early fin buds and these high levels of expression continue until later stages. The T-Box domain binds to double-stranded DNA.