Ventana told its shareholders Tuesday it recommends they "take no action at this time" in response to a $3 billion hostile takeover bid from Swiss drug maker Roche.
Roche offered $75 per share cash, and Ventana's stock soared $28.65, or 55.4%, to $80.39 in premarket electronic trading. The stock closed Monday at $51.74.
Ventana's clinical systems are used to analyze human tissue to help diagnose and treat cancer and infectious diseases.
The company said it will review Roche's offer and will make a recommendation to shareholders within 10 business days after the start of the tender offer.