HIF1A and neoplasm: VSV replication is comparable in hypoxic and normoxic conditions.99 RV reduces HIF-1α expression in hypoxic tumor fractions, contributing to the overall oncolytic effect by reducing transcription of HIF1α-responsive genes including VEGF and those responsible for maintaining CSC phenotypes.100 Many groups have attempted to target hypoxic fractions using hypoxia-specific promoters, however these are often less powerful drivers of gene expression compared with constitutive viral promoters.