SNX18 is specifically and transiently expressed by motor neurons in the embryonic spinal cord. (A) Primary structures of chick and mouse SNX18. The percentage of identical amino acid residues between the structural domains of the chick and mouse orthologs is indicated. (B) SNX18 expression in the embryonic spinal cord. The expression of SNX18 in the brachial-level embryonic spinal cord was observed by in situ hybridization in chick embryos at HH15 (a), HH20 (c), HH23 (e), and HH28 (g) and in mouse at E10.5 (i), E11.5 (k). The expression of Isl1 in the adjacent sections (b, d, f, h, j, l) is shown to indicate the areas where motor neurons reside. The magnified view of the dotted boxes in c and d is indicated in c′ and d′. The outline of the spinal cord is shown by a dashed line in c′ and d′. The expression of SNX18 was downregulated in the chick and mouse motor neurons that moved laterally and became mature. Highest expression of SNX18 at HH23 and HH28 is observed in the region where newly generated motor neurons reside, shown by the expression of Isl1 (e-h, arrowheads). DRG neurons started expressing SNX18 around HH23. Scale bar: 100 µm.