Vertex is planning a major expansion in the Seaport

One of the business pioneers of the Seaport District appears ready to grow in a new corner of the burgeoning neighborhood.

Vertex Pharmaceuticals is poised to lease a building under construction in the Raymond L. Flynn Marine Industrial Park, the biotechnology company said this week. A lease has not been finalized, but Vertex and development firm Related Beal are in advanced talks for 256,000 square feet of space at Innovation Square, a lab and office project Related is building on Tide Street.

Vertex would use it as a research and manufacturing facility for genetic and cellular therapies, a key part of the company’s push into treatments for diseases beyond cystic fibrosis, which has long been its main focus. Earlier this year, the drug maker bought Watertown-based Exonics, which is developing gene therapies for Duchenne muscular dystrophy, and Semma Therapeutics, which is working on medicines for type 1 diabetes. Vertex also is moving forward in clinical trials of gene editing treatments.