Vasa protein expression is restricted to the small micromeres of the sea urchin, but is inducible in other lineages early in development. Voronina, E., Lopez, M., Juliano, C. E., Gustafson, E., Song, J. L., Extavour, C.G., George, S., Oliveri, P., McClay, D., and Wessel, G. Developmental Biology, 314(2): 276-286 (2008)