 In "Action Comics" #261 (February 1960), Supergirl was experimenting on a piece of green kryptonite in an attempt to find a way to neutralize its deadly effects.When her experiment failed, she tossed the kryptonite (or "X-Kryptonite", as it became known) out the window.Though her experiment failed, her pet Streaky came across the X-Kryptonite and was exposed to its radiation.