CXCR4 and Langerhans cell histiocytosis: In the group of patients who presented with LCH at multiple sites (defined as multi-system LCH with and without risk-organ involvement or poly-ostotic LCH), 18/20 patients (90%) displayed CXCR4+ LCH cells at diagnosis (Fig. 4A); 4/6 (66%) poly-ostotic LCH patients in the latter group developed a multi-system variant of LCH during follow-up.