Giving up all its initial gains, the Sensex at the Bombay Stock Exchange stood 77 points, or 0.4 per cent in the negative zone, at 18,716 in afternoon deals, a short while ago. Earlier in the morning, however, the Sensex had opened a marginal 10 points in the green, at 18,804, on selective buying by funds and retail investors.Stock markets in China, South Korea, Taiwan, Japan, Indonesia and Singapore were trading mixed, today. Across in the US, the US Dow Jones Industrial Average had ended flat in yesterday's trade.