Side view of a live 4-day-old Tg(shh;gfp) transgenic zebrafish larva. GFP fluorescence (green) is seen in cells that express the Sonic Hedgehog (Shh) signaling molecule. In the trunk, floor plate cells of the spinal cord express Shh that helps pattern the CNS and muscles. In the head, Shh is expressed in a complex pattern and helps pattern the D/V and A/P axes of the brain, as well as the pituitary gland.