The dominant-negative effect correlated positively with residual ALP activity (<i>r</i> <sub><i>s</i></sub> = 0.889, <i>p</i> < 0.05).<h4>Conclusion</h4>Compound heterozygous <i>ALPL</i> mutations c.299C>T (p.Thr100Met) and c.1271T>C (p.Val424Ala) were causative factors of infantile HPP in the monozygotic twins, providing insights into how dominant-negative effects influence HPP severity. The gene discussed is ALPL; the disease is hypophosphatasia.