The Foreign Institutional Investors (FIIs) were net buyers to the tune of Rs 665.56 crore in the futures & option segment on Friday.According to data released by the NSE, FIIs were net buyers of index futures to the tune of Rs 545.75 crore and bought index options worth Rs 106.15 crore. They were net buyers of stock futures to the tune of Rs 19.62 crore while sold stock options worth Rs 5.97 crore.As per data available on the Sebi website - FIIs were net sellers of stocks worth Rs 469.70 crore in the cash market on Thursday. Mutual funds were net buyers of shares worth Rs 107.77 crore on the same day.