(Reuters) – Merck & Co will buy drugmaker Acceleron Pharma (NASDAQ:) Inc for about $11.5 billion, the companies said on Thursday, as the U.S. pharmaceutical giant looks to beef up its portfolio with drugs for rare diseases.

Merck will pay $180 each Acceleron Share in Cash, which is about 2.6% more than Wednesday’s close price, according to Refinitiv data.

Acceleron is based in Cambridge Massachusetts and focuses on therapeutics that treat blood-related diseases such as cardiovascular disease and diabetes.

Sotatercept is being developed by the company. It is in late-stage clinical trials and is intended to treat rare heart disease, pulmonary arterial hypertension. This is a condition that causes high blood pressure, which affects the lungs.

Drugmakers can make more money by developing rare diseases treatments. GlobalData, an analytics and data company, predicts that the PAH market will grow at a compound annual rate of 5% between 2019-2029 in seven markets.

In 2017, Johnson & Johnson (NYSE:) bought Swiss biotech company Actelion and its pulmonary hypertension drugs in a $30 billion all-cash deal. Merck is working on a treatment to treat this indication. It is in the mid-stage of development. Merck will also have access to Reblozyl. This drug is approved for treatment of blood disorders such as anemia and beta thalassemia.

It is anticipated that the transaction will close during the fourth quarter.