It has also been described for HERV-E.FABP7 in leukemic B cells [18]; HERV-E.PTN, HERV-E.EDNRB, and HERV-E.MID1 in placenta but not in blood cells [41]; and HERV-E clone 4-1 in peripheral blood lymphocytes (CD4+, CD8+ and B cells) but not in neutrophils from patients with systemic lupus erythematosus (SLE) [42,43,44]. Here, PTN is linked to systemic lupus erythematosus.