NanoString Technologies said it would price its follow-on public offering of $55 million of common stock at $18.50 a share. It also granted underwriters an option to purchase up to $8.25 million more of its common stock.

The Seattle company develops life-science research tools, including analytical systems used by cancer researchers to study the activity of genes in small tissue samples. It plans to use the stock offering’s proceeds to commercialize and expand the use of its diagnostic systems, to develop other products and for general corporate purposes.

NanoString stock closed down 38 cents, or 1.9 percent, at $19.56 Thursday before the pricing news was announced.