The FDA approved Mirum Pharmaceuticals’ zilurgisertib for ultra-rare soft tissue disease, marking the third treatment in the space. The therapy targets ALK2 and will be marketed as Atebrioz, providing a new option for patients with a condition that turns muscle and connective tissue to bone. For Mirum, the approval extends the company’s footprint in hereditary musculoskeletal and connective tissue disorders, where trial sizes are small and regulatory pathways often depend on clear mechanistic and clinical signal. The green light also underscores how niche biology—here, ALK2 signaling—continues to translate into approvals in very small patient populations.