The current understanding of development has been shaped by studies in which gene promoters are fused to β-galactosidase (β-gal), or more recently, green fluorescent protein. However, because they depend on light microscopy, these techniques are limited in their ability to probe opaque tissues. Now on page 321, Louie et al. describe a way to use magnetic resonance imaging (MRI) to view domains of gene expression within the interior of a Xenopus embryo. With the goal of tracking gene expression in a living embryo dynamically over time, Louie et al. have developed a contrast agent (1-(2-(β-galactopyranosyloxy)propyl)-4,7,10-tris (carboxymethyl)-1,4,7,10-tetraazacyclododecane)gadolinium(III) (EgadMe), that yields an MR image upon cleavage by β-gal. In the inactivated form, EgadMe's coordination with water is blocked, and thus the molecule is invisible by MRI. β-gal enzymatically cleaves a sugar off the contrast agent, thereby freeing a coordination site for water and converting it to an active, MRI-visible state.