Domestic stock markets opened largely flat on Sunday during a special budget session, as Union Finance Minister Nirmala Sitharaman is set to present her ninth consecutive Union Budget at 11 am. The Nifty 50 index opened at 25,333.75, registering a marginal gain of 13.10 points or 0.05 per cent, while the BSE Sensex opened at 82,445.97, up by 176.19 points or 0.21 per cent.