Erasing all its opening gains, the Sensex at the Bombay Stock Exchange fell 38 points, or 0.2 percent, to trade at 21,155, in the afternoon session, a short while ago. The Nifty at the National Stock Exchnage shed 20 points, or 0.3 percent, to 6,294.In other Asian markets, key benchmark indices in Japan, Hong Kong, China, South Korea and Singapre had been trading mixed, in intra-day trade.