The depletion of stratospheric ozone results in increasedUV (ultraviolet) light below 300 nm, and has signi®cant e ects onbiological the e ectsofUV in this range, early signaling events induced by monochro-maticUV lightwere investigatedusing the chickenBcell line DT40 andmutants lacking protein tyrosine kinases (PTKs). AmongMAPkinase familyproteins, P38MAPkinase (P38) was selectively and immediately activated by 280 nm UV light in cultured DT40 cells.