Snapping two days of gains, the Sensex at the Bombay Stock Exchange fell a modest 21 points, or 0.1 percent, to 16,536, on profit-booking by investors, and amid weak European markets, today. The Sensex had gained 765 points in the previous two trading sessions. The Nifty at the National Stock Exchange lost 5 points, or 0.1 percent, to 4,974. But stock markets in Japan, China, Hong Kong, Singapore and South Korea gained between 0.2 percent and 2.4 percent.