NEW YORK ( ETF Expert) -- Since the summertime lows, a number of themes have gained enormous traction. U.S. real estate is benefiting from ultra-low mortgage rates, limited supply and remarkable demand from overseas buyers.Chinese leadership continues to provide just enough government support to maintain economic targets. And the European Central Bank is having success at containing its sovereign debt woes, in spite of the region's deepening recession. Investors that engaged the trends early have profited immensely from price appreciation in certain ETFs. Since June, for example, the iShares DJ Home Construction Fund ( ITB) has had little difficulty soaring skyward on the notion that a genuine real estate renaissance is in progress. (See the chart below.) However, if fiscal cliff concerns elevate as 2012 draws to a close, you may see investors with large capital gains in the sector take profits. Apple ( AAPL) shareholders have witnessed this activity for more than a month. It follows that the price of ITB may revert to its mean, or 200-day exponential moving average. That's close to 14% drop from present levels. Moreover, we typically think of assets that are 10% above critical trend lines as being "overbought."