ALPL and mucopolysaccharidosis: Consistent with these findings, an enriched immunoreactivity for COL1A1 and other proteins typically expressed during late cartilage hypertrophy and ECM mineralization, such as alkaline phosphatase (ALPL), matrix metalloproteinase 13 (MMP13), and osteocalcin (OCN), was observed in the outer layer of HD pellets but not in MPS IH samples (Figure 3C).