the company for $4.2 billion, attracted by a pipeline of potentially lucrative drugs to treat rare diseases.
The Fed's bond-buying stimulus program has been one of the primary reasons for this year's stocks rally as it has supported the economy and lured investors into riskier assets like equities. The S&P 500 is up 24 percent for the year so far.
Many analysts fear that the market may be ripe for a pullback before the end of the year, especially if the Fed decides to begin reducing its stimulus as early as December. Until then, the trend may remain upward.
"You can't argue with the momentum the market has had thus far, so it looks like we're still in that upward trend," Hellwig said.
Morgan Stanley analysts said in a research note that a sooner-than-expected Fed tapering could hurt defensive shares, which outperformed in October.
Investors also will pay close attention this week to results from retailers, including Macy's and Wal-Mart, both of which are expected to report increases in earnings from a year ago, based on Thomson Reuters data.
Shares of Macy's shot up 1.9 percent to $47.07 on Monday, while shares of Wal-Mart gained 1.4 percent to $79.01.
About 4.78 billion shares traded on the New York Stock Exchange, the Nasdaq and the NYSE MKT, below the five-day average closing volume of about 6.62 billion, according to BATS exchange data.
Advancers outpaced decliners on the NYSE by a ratio of 15 to 14, while on the Nasdaq, decliners outnumbered advancers by about 13 to 12.