sentence: Similarly , BMP4 activity implies important roles for SMADs , and RA activity implies that at least some of the retinoid receptors and metabolizing enzymes [ RAR α , RAR β , RAR γ , RXR α , RXR β , RXR γ , RALDH1 , RALDH2 , RALDH3 , alcohol dehydrogenase , retinol dehydrogenase , cytochrome P-450 ( CYP ) 26A , CYP26B , CYP26C , and stimulated by RA gene 6 ] will have essential functions that still need to be evaluated .