Core Concept: Human artificial chromosomes offer insights, therapeutic possibilities, and challenges.

Francesco Saverio Tedesco works with a big gene. In fact, the human dystrophin gene, with its whopping 2.4 million nucleotides, is one of the largest found so far in nature. A clinician-scientist at University College London, Tedesco hopes to use gene therapy to replace a faulty version of dystrophin in people with Duchenne muscular dystrophy. But the large deletions in some patients are too big to fix with gene editing, and the entire dystrophin gene is too large to fit inside the viruses normally used to deliver replacement DNA. Instead, Tedesco and his colleagues turned to human artificial chromosomes, or HACs, which can hold an essentially unlimited amount of DNA. They have already used HAC delivery to improve movement and other muscular dystrophy features in a mouse model (1), and are now trying it out in human cells. HACs, whether designed from scratch or built using natural human chromosomes as a framework, are slowly gaining in popularity. These artificial DNA molecules, which can exist in human cells as an extra, 47th chromosome, are useful in a variety of applications, from basic studies of gene and chromosome function to potential stem cell and, perhaps, gene therapy treatments. Eventually, some synthetic biologists would like to use HACs to hold entire man-made biological pathways. HACs can also complement the much-discussed CRISPR/Cas9 gene-editing tools, experts say. “CRISPR is good for creating specific and relatively small changes—deletions, insertions, mutations—in an existing genome,” says Alina Chan of HarvardMedical School in Boston. “HACs are for large, megabase-scale genome