HDAC9 and trisomy 21: We previously demonstrated that treatment with the chemical compound 4-PBA, which functions as a chemical chaperon and class I and class IIa histone deacetylase (HDAC) inhibitor, effectively reduced the accumulation of protein aggregates in trisomic human fibroblasts and suppressed the progression of premature senescence in secondary fibroblasts derived from human trisomy 21 iPSCs18.