The Nikkei stock index sank over 2 percent early Thursday, as a stronger yen against the U.S. dollar led to sell-offs of exporter shares and tech issues tracked overnight plunges of their U.S. counterparts. In the first 15 minutes of trading, the 225-issue Nikkei Stock Average fell 917.76 points, or 2.23 percent, from Wednesday to 40,179.93. The broader Topix index was down 40.63 points, or 1.39 percent, at 2,874.58. On the top-tier Prime Market, decliners were led by electric appliance, machinery and transportation equipment issues.