SMN1 and proximal spinal muscular atrophy: To unravel the molecular mechanism of respiratory failure in mutant Zpr1Hb9MNΔ and Zpr1ChATMNΔ mice caused by ZPR1 deficiency and its correlation with respiratory distress in SMA pathogenesis, we examined protein levels of SMN, HoxA5 and HoxC5 in the spinal cords from E12.5 and E18.5 control and mutant embryos.