Merck will buy drugmaker Acceleron Pharma 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.

According to data from Refinitiv, Merck will pay $180 for each Acceleron share in cash. This is a premium that amounts to about 2.6% of Wednesday’s stock closing price.

Acceleron, a company based in Cambridge, Massachusetts, focuses its efforts on developing and marketing therapeutics for cardiovascular diseases and other blood disorders.

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 bought Swiss biotech company Actelion and its pulmonary hypertension drugs in a $30 billion all-cash deal. Merck has also been working to develop a treatment. This is currently in mid-stage. Merck, along with Sotatercept will have access to Reblozyl. It is an approved treatment for two blood-related conditions, including anemia in patients suffering from beta thalassemia.

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