May be a transcription regulator involved in cell-cycle progression and skeletal muscle differentiation. May repress GTF2I transcriptional functions, by preventing its nuclear residency, or by inhibiting its transcriptional activation. May contribute to slow-twitch fiber type specificity during myogenesis and in regenerating muscles. Binds troponin I slow-muscle fiber enhancer (USE B1). Binds specifically and with high affinity to the EFG sequences derived from the early enhancer of HOXC8 (By similarity). Interacts with the retinoblastoma protein (RB1) via its C-terminus. Highly expressed in adult skeletal muscle, heart, fibroblast, bone and fetal tissues. Expressed at lower levels in all other tissues tested. Highly expressed in developing and regenerating muscles, at the time of myofiber diversification. The N-terminal half may have an activating activity. GTF2IRD1 is located in the Williams-Beuren syndrome (WBS) critical region. WBS results from a hemizygous deletion of several genes on chromosome 7q11.23, thought to arise as a consequence of unequal crossing over between highly homologous low-copy repeat sequences flanking the deleted region. Haploinsufficiency of GTF2IRD1 may be the cause of certain cardiovascular and musculo-skeletal abnormalities observed in the disease. Belongs to the TFII-I family.