Under the gun limped? Always makes me suspicious, but then that's because I fold the majority of hands UTG, and I like to limp with aces and kings here, hoping for the late pos raise. So if I see UTG limp in, I tighten my range. Still, I think I raise here to a) push the button off, and b) ask questions of the UTG limper. If he flats, I put him on 22+, Axs, or perhaps decent suited connectors. If he reraises, it's an easy fold.