Generation of C-terminal AAT peptides, such as C37 and C42, depends on specific matrix metalloproteinases (MMPs), which contribute to liver fibrosis; for example, MMP-3 is altered in children with AATD, and MMP-7 is upregulated in pediatric biliary atresia (32). The gene discussed is SERPINA1; the disease is alpha 1-antitrypsin deficiency.