Here’s a common scenario: A couple is having trouble conceiving so they both go to their doctor to see what’s wrong. She gets check out are her ovaries are fine. He gets tested and his sperm count is normal. So what’s the problem? New research suggests that it could be the absence of a protein that coats the surface sperm and helps it travel through fluids in the female reproductive tract.

Reporting online yesterday in the journal Science Translational Medicine, a team led by Gary Scherr and Ted Tollner from the University of California–Davis showed that men with two copies of a mutation in the DEFB126 gene — which encodes a protein normally expressed in the epididymis, where sperm are stored — had a 30% lower chance of successful fertilization.

Above left, you can see healthy sperm with the proper DEFB126 surface coat covered in negatively charged sugars (green) that help sperm penetrate cervical mucus. On the right, sperm with only mutated DEFB126 proteins show fewer charged proteins and have trouble swimming through the mucosal fluids.

To borrow a phrase from Kermit the frog, here’s another reason why it’s good to be green.

Image: Ted Tollner