The iShares Silver Trust (NYSEArca: SLV) was the primary mover in exchange traded funds before the bell Tuesday with a 1.5% gain. Investors will be watching protests and strikes in Greece ahead of votes on austerity measures Prime Minister George Papandreou is trying to implement to receive additional financial support from Europe.

In U.S. stocks, SPDR S&P 500 ETF (NYSEArca: SPY) slipped fractionally in premarket trading while Nike (NYSE: NKE) posted better-than-expected earnings. Investors will get reports after the market opens on U.S. home prices and consumer confidence.

However, investors remain focused on Greece and the European debt crisis. [ETF Spotlight: Vanguard MSCI Europe]

Standard & Poor’s Equity Research this week warned investors to check the exposure to Europe in their ETFs.

S&P cautions investors to understand the risks of owning ETFs that provide Eurozone exposure and the underlying holdings within the funds, particularly sector allocations to financials and banking.

“The potential sovereign risk of default in Greece has led most economists, in our opinion, to be more conservative about the outlook for Europe,” the analysts wrote in a report. “Regulators are also mandating higher capital reserve requirements for major European banks as a result of the past financial crisis. We believe this policy action along with select banks having above average lending exposure to Greece presents some additional risks for investors.”

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