Diagrammatic summary of retinoic acid receptor (rar) expression during development of the European river lamprey, Lampetra fluviatilis, and of the house mouse, Mus musculus. Indicated, in (A), are the expression domains of the four L. fluviatilis rar genes (rar1, rar2, rar3, and rar4) at developmental stages 19, 23, and 26 and, in (B), of the three M. musculus rar genes (rarα, rarβ, and rarγ) at developmental stage E9.5 . Comparisons focus on gene expression in central nervous system (CNS), pharyngeal region, as well as optic and otic territories. See main text for detailed description of rar expression in L. fluviatilis. In mouse embryos at stage E9.5, rarα is expressed in the CNS (in rhombomere 4 and in spinal cord and posterior hindbrain up to the rhombomere 6/7 boundary), in optic and otic territories, as well as broadly, but inconspicuously, in the pharyngeal region. At E9.5, rarβ is expressed in the CNS (spinal cord and posterior hindbrain up to the rhombomere 6/7 boundary), in the posterior portion of the first pharyngeal arch, as well as in tissues surrounding the optic and otic vesicles. At E9.5, rarγ is not transcribed in the CNS, but is broadly expressed in the pharyngeal region as well as the otic territory and is further detectable in tissues surrounding the optic vesicle.