Through extending the number of antibodies for prospective screening of PB or BM samples from newly onset LCH patients, we expect to be able to finally answer the question whether these circulating CD1a+ ‘LCH-like’ myeloid cells are actually mature CD207+ LCH-cells which have re-entered the circulation or whether these cells represent the long sought BM-derived precursor cells which give rise to tissue-resident LCH cells as suggested by Berres et al..6 Here, CD207 is linked to Langerhans cell histiocytosis.