Human CD99, which is encoded by the mic2 gene, is a ubiquitous 32 kDa 
transmembrane protein. Its major cellular functions are related to homotypic 
cell adhesion, apoptosis, vesicular protein transport, and differentiation of 
thymocytes or T cells. Recent reports have suggested that expression of a splice 
variant of CD99 increases the invasiveness of human breast cancer cells. In 
order to determine the structural basis of CD99 function, we have initiated 
structural studies on the human CD99 Type I cytoplasmic domain (hCD99cytoI) 
using circular dichroism and multi-dimensional NMR spectroscopy. The solution 
structure of hCD99cytoI shows that it has a hairpin shape anchored by two 
flexible loops. Consequently, hCD99cytoI does not have any regular secondary 
structural element; however, the NMR and CD data indicate that it possesses an 
intrinsic helical nature.