and early lung branching, but needs to be downregulated for more distal branching and distal airway formation to proceed to completion [Malpel et al., 2000; Wongtrakool et al., 2003]. In rat, there is persistent expression of RARs in the fetal and neonatal lung [Grummer et al., 1994; Grummer and Zachman, 1995], suggesting that RA might also function during lung maturation. Indeed, exogenous RA stimulates alveoli formation in immature rat and mouse lung [Massaro and Massaro, 2000] and murine RARα and RARβ do regulate the septation of alveoli at distinct time points of postnatal lung maturation ([Massaro and Massaro, 2000; Massaro et al., 2003]; also see [Hind et al., 2002; Maden, 2004; Maden and Hind, 2004; Stinchcombe and Maden, 2008]).
