 And so what is he looking for exactly? Is he looking for a virus that's super good at getting into chicken cells and therefore terrible at human cells? Or is he looking for something else? I mean, I think that's sort of the art. This is a judgment call. Hilleman described it as a judgment call. It's guts and judgment. It's just absolute trial and error. I mean, there's no formula for this. It's not written down anywhere. You just try it.