The BSE benchmark Sensex gained over 90 points in opening trade today on the back of selective buying in blue-chips amid covering up of pending short positions by speculators. The 30-share Sensex, which lost 75 points in the previous session on Friday, rose by 90 points, or 0.57 per cent, to 15,829 in opening trade.In a similar fashion, the wide-based National Stock Exchange Nifty index moved up by 25 points, or 0.53 per cent, to 4,739.In Asia, Japan's Nikkei Index rose by 1.37 per cent in morning trade today, while the US Dow Jones Industrial Average ended 1.02 per cent higher in Friday trade.