The demethylase KDM4A (12, 13), previously implicated in HF (16), acts specifically on di- and trimethylated H3K36 and H3K9 (13), which were found to be enriched in the hearts of iron-deficient animals (Figure 1I and Supplemental Figure 1; supplemental material available online with this article; https://doi.org/10.1172/jci.insight.125618DS1). Here, KDM4A is linked to hydrops fetalis.