PAX7 and rhabdomyosarcoma: Disruption in the regulatory circuits controlled by YY1 leads to abnormal myogenic differentiation, underlying the pathogenesis of several muscle diseases such as rhabdomyosarcoma and Duchenne muscular dystrophy.430 Studies have shown that mice lacking YY1 specifically in cells expressing PAX7 (Paired Box 7, a transcription factor involved muscle stem cell proliferation) exhibit impaired diaphragm development, resulting in neonatal death.