Mammalian genome writing: Unlocking new length scales for genome engineering

The ability to design and engineer mammalian genomes across arbitrary length scales would transform biology and medicine. Such capabilities would enable the systematic dissection of mechanisms governing gene regulation and the influence of complex haplotypes on human traits and disease. They would also facilitate the engineering of disease models that more faithfully recapitulate human physiology and of next-generation cell therapies harboring sophisticated genetic circuits. Over the past decade, advances in genome editing have made small, targeted modifications at single sites routine. However, achieving multiple coordinated alterations across long sequence windows (>10 kb) or installing large synthetic DNA segments in mammalian cells remains a major challenge. Recent advances in mammalian genome writing-the bottom-up design, assembly, and targeted integration of large custom DNA sequences, independent of any natural template-offer a potential solution. Here, we review key technological developments, highlight emerging applications, and discuss current bottlenecks and strategies for overcoming them.