Their theory is that this variety of receptor might help, in normal circumstances, to keep control over the brain's response to huge hormonal changes during and immediately after pregnancy.

BBC: Chemical clue to baby blues found

In theory, the medicines could affect the immune system, because a receptor on immune cells is very similar to DPP-4.

FORBES: Magazine Article

One theory is that vitamin D interferes with the effect of the androgen receptor, which is stimulated by hormones such as testosterone and implicated in prostate cancer.

BBC: Vitamin D hope in prostate cancer