(Bloomberg) -- Investors should watch for 10-year Treasuries to move above 2.6%, a threshold that would mark an end to the three-decade bond bull market and be a more important barometer than the Dow Jones Industrial Average passing 20,000, according to billionaire bond manager Bill Gross.

"It is the key to interest rate levels and perhaps stock price levels in 2017," Gross, manager of the $1.8 billion Janus Global Unconstrained Bond Fund, wrote in a monthly investment outlook released Tuesday. "Investment happiness and/or despair may lie ahead over the next 12 months depending on it."

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