The Bombay Stock Exchange benchmark Sensex fell by over 51 points in opening trade today as funds and retail investors locked in gains recorded in the previous session amid a weak trend on other Asian bourses over US debt worries. Marketmen said, the cautious approach adopted by market participants ahead of Sensex heavyweight RIL's Q1 results announcement and the RBI's monetary policy review tomorrow cast a shadow on the trading sentiment. The 30-share Sensex, which gained over 286 points in the previous session, fell by 0.27 per cent, to 18,670 in the first few minutes of trade. In a similar fashion, the wide-based National Stock Exchange Nifty index shed over 17 to 5,616 points. Meanwhile, in the Asian region, Hong Kong's Hang Seng Index was down 0.90 per cent, while Japan's Nikkei lost 0.88 per cent in morning trade today. The Dow Jones Industrial Average in the US ended 0.34 per cent lower in the previous session on Friday.