Why are nude mice nude?

Personally, I think nude mice are super-cute!

Nude mice, commonly used in research, are instantly identifiable by their hairless, wrinkled pink forms.  Sometimes called “athymic”, due to their lack of a functional thymus, nude mice are also immunodeficient and do not produce T cells.  But how do immunodeficiency and hairlessness relate?

Nude mice have a mutation in a gene called FOXN1.  This gene encodes a transcription factor, which is a protein that is required to activate a gene.  In this case, FOXN1 normally encodes a protein which activates a gene involved in the differentiation of a type of cell called epithelium.

Epithelial cells are cells which form body surfaces, such as skin, and glands, such as the thymus.  Normal development of epithelial cells results in skin with hair and a functional, T cell producing thymus.

The mutation in FOXN1 disrupts the normal development of both skin progenitor cells and thymus progenitor cells.  This results in skin which forms without hair and a non-functional thymus.  And there you go!  A nude, immunodeficient mouse!