HOXA1 and Respiratory distress: The increased ratio of Firmicutes to Bacteroidetes; the higher relative abundances of Firmicutes, Proteobacteria, and Lactobacillus; and the lower relative abundance of Bacteroidetes were observed in the meconium of the small intestine of Hoxa1–/– neonatal piglets, and the symptom of respiratory distress was not observed in Hoxa1–/– neonatal piglets delivered by sows with ATRA administration.