"Isogenic" cells are cells that are genetically identical except for one specific difference, like twin siblings where one has a targeted mutation and the other doesn't. Researchers create these by editing one copy of the gene in an iPSC line.
Why it matters for the MEF2C microglia papers: The Hansen et al. and Cell Immunity studies used isogenic MEF2C-haploinsufficient and MEF2C-knockout iPSC-derived microglia. This means they could compare MEF2C-deficient microglia against nearly-identical control microglia, proving that any differences were due to MEF2C loss and not other genetic variability. This is the gold standard for establishing causation in cell biology.