Impish. That's great news. Looking forward to hear what you can dig up. Now that you said that, I did a quick pubmed search, and I found a couple of studies on this. Problems with CD57 was associted with CFS. Like I suspected. www.ncbi.nlm.nih.gov/pubmed/1337560 " (...) the mean number of NK cells was lower (p < 0.005) in the patient group, when CD57 was used as the NK cell marker." p < 0.005 means that it's pretty probalbe that it's so... www.ncbi.nlm.nih.gov/pubmed/1849315 "Natural killer cells as defined by CD16, CD56 and CD57 antigens were significantly reduced in CFS." I guess that CFS patients get treated [for XMRV], these CD57 levels will rise, just like CD4 levels do when an HIV infection is treated.