The problem. The same question sat in front of two labs at once: does the bacterial Cas9 system, reprogrammed with a guide RNA, actually edit the human genome in living cells? A single group showing it could be a special case. Two groups showing it independently makes it a method.

The idea. The authors expressed human-codon-optimized Cas9 with short guide RNAs targeting native human genes. Cas9 cut at the programmed sites, and repair produced insertions and deletions at the cut. They tuned the guide design, showed editing at several loci, and demonstrated that a supplied DNA template can be used to write in a chosen edit through homology-directed repair, not only to knock a gene out.

Why it matters. This paper and Cong 2013 landed in the same issue and confirmed each other. Read together, they are the moment CRISPR became a standard human-cell tool. The homology-directed-repair angle matters for my own reading arc: it is the bridge from cutting DNA to writing a specific change, which is the thread that runs through base and prime editing later in this batch.

Verdict. A co-founding method paper for mammalian CRISPR editing. Read it as the confirmation half of the pair, and for the early guide-design lessons.