Forget the intricacies of molecular biology -- the most elusive mystery to haunt Seattle's largest independent biotech company is the true identity of the gorilla that has appeared every Halloween afternoon for at least 15 years.

On Wednesday, the gorilla struck again, wading through the company's fancy Eastlake headquarters and aggressively pushing candy at the employees, undeterred by the flurry of activity surrounding the company's quarterly earnings call.

The unidentified person, about 5' 7", wears a full-body gorilla costume and hunches over like an ape, and is described as very agile and strong.

In all these years, nobody has figured out who it is, says ZymoGenetics spokeswoman Susan Specht. "All theories have come to nothing," she said.

One thing is well known, though: the gorilla doesn't take no for an answer. "I didn't really want candy four years ago, then I quickly learned through sign language that I had to have candy," said Specht.