Real estate-related exchange-traded funds were among the worst decliners Wednesday, as subprime woes continued to swell.

On Wednesday, Standard & Poor's said it could cut its rating on $12 billion in securities backed by subprime residential loans, which raised concerns about the ongoing effects of the housing meltdown.

The iShares FTSE NAREIT Retail ( RTL) ETF was losing $1.29, or 3%, to $42.21. The iShares FTSE NAREIT Real Estate 50 ( FTY) ETF was down 60 cents, or 1.3%, to $44.79. The Ultra Real Estate ProShares ( URE) was off 54 cents, or 1.1%, to $49.91.

Bundled securities tracking semiconductors were also under pressure. Intel ( INTC), Maxim Integrated Products ( MXIM), Texas Instruments ( TXN) and KLA-Tencor ( KLAC) were losing 1% or more.

Among ETFs, the Ultra Semiconductor ProShares ( USD) was losing $1.13, or 1.3%, to $88.28. The SPDR S&P Semiconductor ( XSD) ETF was down 35 cents, or 0.6%, to $55.03. The Semiconductor HOLDRs ( SMH) was off 24 cents, or 0.6%, to $38.88.

Also trading lower were commodity-related ETFs, as prices for crude oil, gold and silver were on the decline.

The Market Vectors Gold Miners ( GDX) was lower by 29 cents, or 0.7%, to $40.37. The iShares Silver Trust ( SLV) was down 20 cents, or 0.2%, to $128.50. The Oil Services HOLDRs ( OIH) was falling by $1.78, or 1%, to $177.90.

On the winning side, ETFs related to basic materials were the best performers. The Ultra Basic Materials ProShares ( UYM) ETF was higher by $1.80, or 2%, to $94.20. The iShares Dow Jones U.S. Basic Materials ( IYM) ETF was adding 84 cents, or 1.2%, to $72.35.

The B2B Internet HOLDRs ( BHH) was also gaining ground, thanks to a 1.2% rise in holding Checkfree ( CKFR). The ETF was tacking on 3 cents, or 1.3%, to $2.36.

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