Tacrolimus (FK506) is a 822 kDa lipophilic macrolide lactone antibiotic with potent immune suppressive activity which has recently been used to treat female infertility in obese and diabetic mice29 and women with recurrent implantation failure with elevated systemic Th1 (CD4+ IFNγ+): Th2 (CD4+IL4+) cell ratios30. This evidence concerns the gene CD4 and female infertility.