* Amazon joins club of 10 largest U.S. companies
* Biogen's fall weighs on Nasdaq, S&P
* Clinton to propose raising capital gains tax - report
* Indexes down: Dow 0.9 pct, S&P 1.1 pct, Nasdaq 1.1 pct (Updates to close)
By Rodrigo Campos
July 24 (Reuters) - The S&P 500 and Nasdaq posted their largest weekly drops since March on Friday as slowing global growth dragged commodity-related stocks lower while an earnings-fueled drop in Biogen took down the biotech sector.
Biogen more than halved its revenue growth forecast for 2015 and its stock lost 22 percent to close just above $300.
Stocks in the energy and materials sectors weighed heavily on the market after weaker-than-expected economic data from China and the euro zone raised concerns about global growth. Oil prices hit their lowest since March.
"You got underwhelming revenue growth on balance and then you layer on top of that concern over a global economic slowdown, that becomes self-fulfilling," Art Hogan, chief market strategist at Wunderlich Securities in New York, said of the stock market weakness.
Amazon was the bright spot on Friday, rising more than 20 percent at one point and closing up 10 percent at $530.50. The online retailer posted an unexpected quarterly profit and its market cap ballooned to $247 billion, making it the tenth-largest U.S. company by market value.
However, Amazon's spike highlighted the thinning of leadership in the S&P 500. Gains on the index so far this year can be attributed to Amazon and just three more companies.
At the close of trading on Friday, the Dow Jones industrial average fell 163.39 points, or 0.92 percent, to 17,568.53, the S&P 500 lost 22.5 points, or 1.07 percent, to 2,079.65 and the Nasdaq Composite dropped 57.78 points, or 1.12 percent, to 5,088.63.
For the week, the S&P fell 2.2 percent and the Nasdaq slid 2.3 percent in their largest weekly drops since the last week of March. The 2.9 percent fall on the Dow was the largest for any week since January.
Selling in stocks accelerated after a report from the Wall Street Journal said Democratic presidential candidate Hillary Clinton will propose nearly doubling the U.S. capital gains tax rate on short-term investments.
Second-quarter S&P 500 earnings have been mixed, with 74 percent of companies beating analysts' profit expectations but just 52 percent surpassing revenue expectations, according to Thomson Reuters data.
Adding to the concerns regarding lukewarm earnings, the S&P 500 is relatively expensive, trading at 16.9 times forward 12 months' earnings, above the 10-year median of 14.7 times, according to StarMine data.
Dow component Visa ended up 4.3 percent at $74.80 after the credit and debit card company's results handily beat expectations.
Declining issues outnumbered advancing ones on the NYSE by 2,246 to 831, for a 2.70-to-1 ratio on the downside; on the Nasdaq, 2,144 issues fell and 660 advanced for a 3.25-to-1 ratio favoring decliners.
The benchmark S&P 500 index posted 13 new 52-week highs and 54 new lows; the Nasdaq Composite recorded 50 new highs and 200 new lows.
Some 7.3 billion shares changed hands on U.S. exchanges, above the daily average of 6.6 billion so far this month. (Reporting by Rodrigo Campos; Editing by Meredith Mazzilli)