The FBN1 gene encodes which connective tissue protein?

• A. Fibrillin-1
• B. Collagen type I
• C. Elastin
• D. Fibronectin

Answer: (A)

FBN1 encodes fibrillin-1, a large glycoprotein that assembles into microfibrils in the extracellular matrix. These microfibrils create a scaffold that supports elastic fiber formation and tissue elasticity, which is why fibrillin-1 is essential for the integrity of connective tissues such as the aorta, ligaments, and eyes. Mutations in FBN1 disrupt this microfibrillar network, leading to disorders like Marfan syndrome.

Collagen type I is the main structural protein of bone, skin, and tendon and is produced by different genes (not FBN1). Elastin is the protein providing elasticity in elastic fibers, but it is deposited on a microfibrillar scaffold; the FBN1 product specifically is fibrillin-1 that forms those microfibrils. Fibronectin is a cell-adhesion glycoprotein involved in ECM interactions, distinct from fibrillin-1.