BST2 and cyclic hematopoiesis: Taken together, these results reveal that (i) BST2 can be considered as a worthy identifying marker for CH cells, thus confirming the microarray data, and (ii) BST2pos CH cells migrate to and engraft in wounded skeletal tissues only when transplanted during the acute phase response, in agreement with the concept that factor(s) released during this period are directly involved in progenitor cell activation.