The foreign institutional investors (FIIs) were net sellers to the tune of Rs 637.80 crore in the futures & option segment on Monday.According to data released on the NSE website, FIIs were net sellers of index futures to the tune of Rs 732.74 crore while they bought index options worth Rs 107.37 crore. They were net sellers of stock futures to the tune of Rs 17.33 crore while they bought stock options worth Rs 4.90 crore.As per data available on the Sebi website - FIIs were net sellers of stocks worth Rs 343.60 crore in the spot market on Friday. Domestic mutual funds (MFs) were net sellers of shares worth Rs 24.14 crore on the same day.