Figure 1: Non-activated or Activated State of Sonic Hedgehog Signaling Pathway.

Left panel: With the absence of HH ligand, PTCH1 inhibits SMO activity. GLI activity is phosphorylated, converting GLI full- length (GLIFL) to repressor form (GLIR). GLIR translocates into the nucleus, which binds to HH target gene promoters and suppresses their expression. Right panel: The activation of the signaling occurs when HH ligands bind to PTCH, HH relieves the inhibition of PTCH to activate the signal transduction. SMO transmits a signal to the cytoplasm in a phosphorylation cascade leading translocation of GLI activator (GLIA) to the nucleus and binding to the target gene promoters and activates the transcription of the target genes.