C4.4A　is　a　glycosylphosphatidylinositol-anchored　membrane　protein　comprised　of　two　LU　domains　(Ly6/uPAR-like　domains)　and　an　extensively　O-glycosylated　C-terminal　Ser/Thr/Pro-rich　region.　C4.4A　is　a　novel　biomarker　for　squamous　epithelial　differentiation.　Its　expression　is　dysregulated　under　various　pathological　conditions　and　it　is　a　robust　biomarker　for　poor　prognosis　in　various　malignant　conditions　such　as　pulmonary　adenocarcinoma.　To　facilitate　crystallization,　the　two　LU　domains　were　excised　from　intact　C4.4A　by　limited　proteolysis,　purified　and　crystallized　by　the　sitting-drop　vapour-diffusion　method.　The　crystals　diffracted　to　2.7　angstrom　resolution　and　belonged　to　space　group　C222(1),　with　unit-cell　parameters　a　=　55.49,　b　=　119.63,　c　=　168.54　angstrom.　The　statistics　indicated　good　quality　of　the　data,　which　form　a　solid　basis　for　the　determination　of　the　C4.4A　structure.