Why Gilead Sciences (GILD) Stock Is Down Today (Update)

Update (4:35 p.m.): Updated with Monday market close information.

NEW YORK (TheStreet) -- Gilead Sciences  (GILD) fell Monday amid news that Merck  (MRK) would purchase biotechnology company Idenix Pharmaceuticals  (IDIX) for $3.85 billion as it tries to strengthen its position in the increasingly competitive hepatitis C sector.

Gilead is currently the leader in the hepatitis C drug market with its pill Sovaldi. Merck's acquisition of Idenix could provide stronger competition for Gilead.

Merck will pay $24.50 a share in cash for Idenix, more than triple the company's closing price of $7.23 on Friday.

Must Read: Merck Buys Idenix for $3.9B in Yet Another Blockbuster Hep C Deal

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"Idenix has established a promising portfolio of hepatitis C candidates,'' said Dr. Roger Perlmutter, the president of Merck Research Laboratories, in a statement Monday. 

Gilead closed down 4.11%, or $3.39, to $79. More than 23.2 million shares changed hands, which beat the average volume of 16,490,200.

For more on this story, read TheStreet's Adam Feuerstein's article here.

GILD Price Chart

GILD Price data by YCharts

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