TWSG1 and embryonal carcinoma: To test potential mechanisms underlying the acute sensitivity of Twsg1 mutant mice to ATRA, we selected P19 mouse embryonic carcinoma cells as an experimental system because they resemble embryonic cells, represent a homogenous cell population that is amenable to quantitative assays and have been used by others as a model for BMP-retinoid signaling interactions (Fujita et al., 1999; Glozak and Rogers, 1996; Glozak and Rogers, 1998).