A study from Weill Cornell Medicine and The Jackson Laboratory has described the epigenetic mark SARS-CoV-2 left on immune system stem cells in the most severe cases of COVID-19 early in the pandemic, before the development of vaccines. In their work published in Cell on Aug. 18, 2023, the researchers presented a new methodology to analyze the epigenetic changes in monocytes and circulating hematopoietic stem and progenitor cells (HSPCs) that give rise to monocytes. That allowed corresponding author Steven Josefowicz and his colleagues to see if there were already changes induced by COVID-19 before HSPCs differentiated into monocytes.