Shares of Illumina, Inc. (ILMN - 52.99) rallied yesterday, after Swiss drug maker Roche Holding once again increased its buyout bid offer -- this time, to roughly $6.5 billion, or $51 per share. As a result, roughly 40,000 calls crossed the tape, which more than tripled the equity's average daily volume. Close to 13,800 of these calls were exchanged at the out-of-the-money April 55 strike. Open interest on this option soared by 11,727 contracts overnight, indicating that a large portion of the volume consisted of new positions. This strike is now home to peak call open interest of 31,108 contracts.

Digging deeper into the data, however, it appears that a block of 5,928 puts was sold at the April 45 strike, while an equal number of puts were simultaneously bought at the April 49 strike. Because both options saw an overnight rise in open interest, it can be assumed that one trader initiated a bear put spread on ILMN. In order to profit from this strategy, the investor needs the stock to close at or below $45 by the time front-month options expire. His maximum reward would be limited to $3.40, or the difference between the strike prices, minus the net debit of $0.60. Conversely, his potential risk is capped at the net debit paid.

It should be mentioned that short interest on the gene-sequencing firm edged up by 2.36% during the past month, and now represents 21.51% of ILMN's float -- or nearly 17 days' worth of pent-up buying pressure, at the stock's average pace of trading. This could mean that some of Thursday's call volume is attributable to short sellers looking to hedge their bearish bets.

Technically, ILMN has gained an impressive 73% year-to-date, and has outpaced the broader S&P 500 Index (SPX) by close to 43% during the last 60 sessions. On the charts, the stock remains perched atop its 200-day moving average, a trendline it surmounted after gapping 46% higher on Jan. 25, when Roche made its initial takeover bid.

In the first hour of the session, ILMN is ahead by over 1% to trade at $52.99.