Illumina Inc. is granting investors the right to buy shares at half the price in a so-called poison- pill defense, following Roche Holding AG’s hostile $5.7 billion bid for the maker of gene-mapping tools.

Shareholders will receive one preferred stock purchase right as a dividend for each common share held as of the close of business Feb. 6, San Diego-based Illumina said in a statement. Investors will also have the right to purchase $550 in common shares for $275, or, assuming a price of $55 a share, 10 common shares at that price. The rights won’t be exercisable initially, Illumina said. The strategy can block an unwanted bid by making it prohibitively expensive.