Snapping four days of losses, the Sensex at the Bombay Stock Exchange gained 51 points, or 0.3 percent, to close at 17,542, on fag-end buying, amid the expiry of the month's derivatives contracts, today. The Sensex had lost 360 points in the last four trading sessions. The Nifty at the National Stock Exchange rose 27 points, or 0.5 percent, to 5,315. But stock markets in Japan, Hong Kong, South Korea and Singapore lost about 1 percent, each.