Rockville-based biotechnology firm GenVec announced yesterday its acquisition of a New Jersey company that would bolster its position in developing gene therapies to treat disease.

GenVec Inc. said it plans to buy biotechnology research and development firm Theragen Inc. of Prince- ton, N.J., for an undisclosed sum.

Both companies are working on ways to engineer viruses so they can deliver doses of medication or new genetic instructions to diseased cells.

Thomas D'Alonzo, chief executive of GenVec, said the two companies use different techniques in their gene therapy products.

"We believe no one [technique] will dominate, we're going to need several for different applications," he said.

Mr. D'Alonzo said the purchase would be in the form of a stock swap between the privately held companies. GenVec will then look to put together an initial public offering of stock at "the earliest opportunity," he said.

Theragen's operations will be moved to GenVec's headquarters in Rockville. Mr. D'Alonzo said that with its acquisition and resulting growth, he expected company employment in Maryland to grow to 50 people, from 22 now, over the next two years.

Company executives said GenVec will continue developing gene therapies to fight cancer as well as diseases of the heart and lungs.