GSC and microcephaly: To address the missing phenotype we must remind ourselves that the Gsc+/Cyp26A1 model is an acute RA deficiency at early gastrulation, not a chronic or repeated acute PAE exposure as seen in studies which show microcephaly as a PAE phenotype (Parnell et al., 2009; Godin et al., 2010a; Lipinski et al., 2012).