PARIS (Reuters) -French pharma group Sanofi has agreed to buy US-based Blueprint Medicines Corporation, a biopharmaceutical company specializing in systemic mastocytosis, a rare immunological disease, the companies said on Monday.

Under the terms of the acquisition, Sanofi will pay $129.00 per share in cash, representing an equity value of approximately $9.1 billion.

The acquisition "represents a strategic step forward in our rare and immunology portfolios. It enhances our pipeline and accelerates our transformation into the world's leading immunology company," said Sanofi CEO Paul Hudson.

The deal will add to Sanofi's portfolio the rare immunology disease drug Ayvakit/Ayvakyt (avapritinib), approved in the US and the EU, and a promising advanced and early-stage immunology pipeline.

Blueprint's established presence among allergists, dermatologists, and immunologists is also expected to enhance Sanofi's growing immunology pipeline, the companies said.

Ayvakit/Ayvakyt is the only approved medicine for advanced and indolent systemic mastocytosis, a rare immunology disease, which is characterized by the accumulation and activation of aberrant mast cells in bone marrow, skin, the gastrointestinal tract, and other organs.

The acquisition will also bring elenestinib, a next-generation medicine for systemic mastocytosis, as well as BLU-808, a highly selective and potent oral wild-type KIT inhibitor that has the potential to treat a broad range of diseases in immunology.

Besides $129.00 per share in cash at the closing of the deal, Blueprint shareholders will also receive one non-tradeable contingent value right (CVR) which will entitle the holder to receive two potential milestone payments of $2 and $4 per CVR for the achievement, respectively, of future development and regulatory milestones for BLU-808.

The total equity value of the transaction, including potential CVR payments, represents approximately $9.5 billion on a fully diluted basis.

Hudson said the deal complements recent acquisitions of other early-stage medicines that remain Sanofi's main field of interest and added that Sanofi still retains a sizeable capacity for further acquisitions.