This is a Phase 2, multicenter, open-label, extension (OLE) study to evaluate the long-term safety, tolerability, and efficacy of infigratinib, an FGFR 1-3-selective tyrosine kinase inhibitor, in subjects with ACH who previously completed a QED-sponsored interventional study, and potentially in additional subjects who are naïve to infigratinib treatment. Quality of Life assessments for this subject population will also be evaluated. Treatment-naïve subjects must have at least a 6-month period of growth assessment in study QBGJ398-001 (PROPEL) and will be enrolled in this OLE study only after a dose to be explored further is identified in Phase 2 Study QBGJ398-201 and subjects are not otherwise eligible to enroll in another QED-sponsored Phase 2 or Phase 3 ACH study.