<p>(A and C) <i>Tbx15</i>. (B and D) <i>Agouti</i>. At E12.5, expression of <i>Tbx15</i> in dorsal skin is approximately complementary to that of <i>Agouti</i> in ventral skin. At E14.5, the levels of expression for both genes are lower, but <i>Tbx15</i> expression has expanded ventrally and overlaps extensively with that of <i>Agouti</i>. In all four panels, arrows mark the approximate ventral limit of <i>Tbx15</i> and the approximate dorsal limit of <i>Agouti</i> (scale bars = 500 μm).</p