It’s not misspelled. The “Shh” refers to Sonic hedgehog, one of the most developmentally significant morphogens involved both in full body patterning and in brain development. It’s secreted by the zone of polarizing activity (Spemann organizer). Loss of Shh dorsalizes the embryo, guides axonal growth…oh, right. This is the Internet. If you’re still reading this, why not try pubmed.org?
(Biologists love acronyms. Just ask a geneticist about BOSS.)