NR4A1 and metastatic neoplasm: Altogether, these results indicate that hormonal receptors play a role in MMTV presence in the blood, which is conceivably attributable to increased viral production through the activation of estrogen-responsive elements in MMTV-LTR region in mammary cells [42,43,44] and that circulating tumor cells in metastatic tumors may also carry MMTV-like sequences in the blood.