Some studies found that the proportion of CD56 + CD16– NK cells in the endometrium in the secretory phase was similar in women with unexplained infertility, in CE and control subjects [147,148]; other studies have described significantly higher levels of CD56 + CD16– NK cells in the endometrium of women with CE compared with those without CE [149,150,151,152,153]. The gene discussed is NCAM1; the disease is cholesteryl ester measurement.