I found the original form of this recipe when looking for a great fajita marinade. It was so incredibly delicious that I started using variations of it on all my grilled meats. It's incredibly fast, and doesn't have a strong flavor of its own. It just seems to make all meat taste juicier, better and more like itself. Any high-acid citrus works well in this recipe, so substitute lemons if you don't have limes. I haven't tried grapefruit yet, but I'm sure it would be interesting. I've also augmented with rice or cider vinegar when my citrus has been a bit dry and that has worked well too. Nothing in this recipe is too exact, and you don't have to be either. The basic combination is just so incredible (and forgiving!), I highly recommend it.