It's possible that:

* There's a typo: Perhaps you meant a different gene, like "Dlx" (Distal-less homeobox genes), which are crucial for development in vertebrates.

* It's a specialized term: If "D1x" is a more specific term used in a particular research field, please provide more context.

If you can clarify the term or provide more details, I can help you understand its regulation and expression.