A single letter mistake in the gene for FBN1, which codes for the fibrillin protein, can cause a ripple effect of problems—from loose joints to weak vision to life-threatening tears in the heart's walls. Starting with healthy eggs and sperm donated by a Marfan syndrome patient, the team of researchers from Shanghai Tech University and Guangzhou Medical University used an IVF technique to make viable human embryos. Then they injected the embryos with a Crispr construct known as a base editor, which swaps out a single DNA nucleotide for another—in this case, removing an "A" and replacing it with a "G". They kept the embryos alive for another two days in the lab, long enough to run tests to see how well the editing worked. Sequencing revealed that all 18 embryos had been edited, with 16 of the embryos bearing only the corrected version of the FBN1 gene. In two of the embryos, additional unwanted edits had also taken place.