CD200 and plasma cell myeloma: Since overexpression of CD200 was reported in a variety of human tumors including multiple myeloma (68), neuroendocrine tumors (69), melanoma (70, 71), ovarian cancer (72), and very recently also in more than 90 % of NB samples (38), high correlation of CD200 and HERV-K GAG for the studied NB cell lines (r = 0.92) might be of special interest (Figure 2).