Acceleron net loss for the first quartere 2019 $38.1M, R&D expenses $32.8

– Luspatercept Biologics License Application (BLA) and Marketing
Authorization Application (MAA) submitted in April 2019 –

– Part 2 results from the ACE-083 Phase 2 trials in
facioscapulohumeral muscular dystrophy (FSHD) expected in the second
half of 2019 and Charcot-Marie-Tooth disease (CMT) anticipated in Q1
2020 –

– PULSAR and SPECTRA Phase 2 trials of sotatercept in pulmonary
arterial hypertension (PAH) on track with topline results expected from
PULSAR in 1H 2020 –

CAMBRIDGE, Mass.–(BUSINESS WIRE)–Acceleron Pharma Inc. (Nasdaq:XLRN), a leading biopharmaceutical company
in the discovery and development of TGF-beta superfamily therapeutics to
treat serious and rare diseases, today provided a corporate update and
reported financial results for the first quarter ended March 31, 2019.

“With the submission of marketing applications for luspatercept in the
U.S. and E.U. last month, we and our global collaboration partner,
Celgene, are excited about the potential to bring a new therapy to
patients with myelodysplastic syndromes and beta-thalassemia within the
next year,” said Habib Dable, President and Chief Executive Officer of
Acceleron. “At the same time, our pulmonary program remains on track,
with enrollment ongoing in two Phase 2 trials of sotatercept in PAH, and
we are anticipating topline results from the placebo-controlled part of
the Phase 2 trial of our locally-acting muscle agent, ACE-083, in FSHD
during the second half of this year.”

Development Program Highlights

Hematology

Luspatercept: Myelodysplastic Syndromes (MDS),
Beta-Thalassemia, and Myelofibrosis (MF)

Luspatercept is an investigational first-in-class erythroid
maturation agent (EMA) designed to address a late-stage erythroid
maturation defect that results in chronic anemia and the need for
regular red blood cell transfusions in adults with serious hematologic
diseases. Luspatercept is part of the global collaboration between
Acceleron and Celgene.

• • Celgene recently submitted a Biologics License Application (BLA) to
    the U.S. Food and Drug Administration (FDA) and a Marketing
    Authorization Application (MAA) to the European Medicines Agency (EMA)
    for luspatercept in patients with MDS- and beta-thalassemia-associated
    anemia based on the safety and efficacy results of the pivotal Phase 3
    studies MEDALIST and BELIEVE.

• • The Companies expect to announce preliminary topline results from the
    Phase 2 trial of luspatercept in patients with MF in the second half
    of 2019.

• • Enrollment is ongoing in the COMMANDS Phase 3 trial in patients with
    first-line lower-risk MDS and the BEYOND Phase 2 trial in patients
    with non-transfusion-dependent beta-thalassemia, with preliminary
    results expected from the BEYOND trial in 2020.

Neuromuscular Disease

ACE-083: Facioscapulohumeral Muscular Dystrophy
(FSHD) and Charcot-Marie-Tooth Disease (CMT)

ACE-083 is an investigational locally-acting therapeutic designed to
have a concentrated effect on muscle mass and strength in target muscles
for diseases that cause focal muscle weakness. ACE-083 utilizes the
“Myostatin+” approach to inhibit multiple TGF-beta superfamily ligands
involved in muscle formation.

• • Previously presented results from Part 1 of the Phase 2 trial
    evaluating ACE-083 in patients with FSHD were highlighted in an encore
    presentation at the Muscular Dystrophy Association (MDA) Clinical &
    Scientific Conference in April 2019.

• • Preliminary results from Part 2 of the Phase 2 trial in patients with
    FSHD are expected in the second half of 2019.

• • Previously presented results from Part 1 of the Phase 2 trial
    evaluating ACE-083 in patients with CMT will be highlighted in a
    platform presentation at the American Academy of Neurology (AAN) 71^st
    Annual Meeting on May 10, 2019.

• • Enrollment is ongoing in Part 2 of the Phase 2 trial in patients with
    CMT, with preliminary results expected in the first quarter of 2020.

Pulmonary Disease

Sotatercept: Pulmonary Arterial Hypertension
(PAH)

Sotatercept is an investigational agent designed to be a selective
ligand trap for members of the TGF-beta superfamily to rebalance BMPR2
signaling, which is a key molecular driver of PAH. In preclinical
studies of PAH, sotatercept reversed pulmonary vessel muscularization
and improved indicators of right heart failure.

• • Enrollment is ongoing in the PULSAR Phase 2 trial in patients with
    PAH, with topline results expected in the first half of 2020.

• • Enrollment is ongoing in the exploratory SPECTRA trial in patients
    with PAH, with preliminary results expected in 2020.

• • A preclinical abstract of sotatercept in PAH has been accepted for
    presentation at the American Thoracic Society (ATS) 2019 International
    Conference on May 21, 2019.

Financial Results

• • Cash Position – Cash, cash equivalents and investments as of
    March 31, 2019 were $513.1 million. As of December 31, 2018, the
    Company had cash, cash equivalents and investments of $291.3 million.
    Based on the Company’s current operating plan and projections, it
    believes that current cash, cash equivalents and investments will be
    sufficient to fund projected operating requirements until such time as
    it expects to receive significant royalty revenue from luspatercept
    sales.

• • Revenue – Collaboration revenue for the first quarter was $2.8
    million. The revenue is all from the Company’s partnership with
    Celgene and is primarily related to expenses incurred by the Company
    in support of luspatercept.

• • Costs and Expenses – Total costs and expenses for the
    first quarter were $43.6 million. This includes R&D expenses of $32.8
    million and G&A expenses of $10.8 million.

• • Net loss – The Company’s net loss for the first quarter
    ended March 31, 2019 was $38.1 million.

Conference Call and Webcast

The Company will host a webcast and conference call to discuss its first
quarter 2019 financial results and provide an update on recent corporate
activities on May 9, 2019, at 5:00 p.m. EDT.

The webcast will be accessible under “Events & Presentations” in the
Investors/Media page of the Company’s website at www.acceleronpharma.com.
Individuals can participate in the conference call by dialing
877-312-5848 (domestic) or 253-237-1155 (international) and referring to
the “Acceleron First Quarter 2019 Earnings Call.”

The archived webcast will be available for replay on the Acceleron
website approximately two hours after the event.