Shares of DocuSign Inc. (NASDAQ: DOCU), a leading provider of electronic signature solutions, surged over 8% in pre-market trading on Monday. The stock's rally was driven by the announcement that the company will be added to the S&P MidCap 400 index, effective October 11th.

The S&P Dow Jones Indices revealed late on Sunday that DocuSign will replace MDU Resources Group Inc. (NYSE: MDU) in the S&P MidCap 400. This move comes as MDU Resources announced its intention to spin off a new company later this month, prompting its shift to the S&P SmallCap 600 index.

DocuSign's inclusion in the S&P MidCap 400 is expected to drive increased demand for the stock from index funds and exchange-traded funds (ETFs) that track the index. As these funds rebalance their portfolios to reflect the index changes, they will need to purchase shares of DocuSign, potentially boosting the stock's price.