The CRISPR system enables researchers to make a small chain of custom-made molecules, called a guide RNA, and a Cas9 enzyme. The guide RNA is like the search function of a word processor, running along the length of the genome until it finds a match; then, the scissorslike Cas9 cuts the DNA. CRISPR can be used to delete, insert, or replace genes.

"We didn't used to think that we had the tools to correct mutation in humans," said Penn Medicine cardiologist Jonathan Epstein, who just began using the technique in his lab. "The advantage of CRISPR is that we can."

For instance, sickle-cell anemia is caused by a mutation in chromosome 11 that causes red blood cells to be crescent-shaped, sticky, and stiff. They end up stuck in the blood vessels, keeping enough oxygen from reaching the body. While the disease can be treated with bone marrow or stem cell transplants, most patients cannot find well-matched donors.

Here's where CRISPR can help. Biomedical engineer Gang Bao of the Georgia Institute of Technology aims to use the system to repair the DNA of a patient's own stem cells, so no outside donor would be needed. The stem cells would be extracted from the patient's bone marrow, their mutations replaced with normal DNA, and inserted back in. The hope is that the gene-corrected stem cells would then begin making normal red blood cells.

The treatment works in mice, and Bao foresees human trials within a few years.

Another way doctors could use CRISPR is to assist in regenerating tissue within damaged organs. Epstein ultimately wants to place embryonic stem cells that have developed into cardiac muscle cells back into the heart. But the main danger with this lies in accidentally injecting any non-cardiac cells. "If you put a cell into the heart meant to make a tooth or a hair, it might cause a tumor," said Epstein.

So instead of blindly inserting a group of cells hoping they are all cardiac muscle, he is using CRISPR to insert marker genes - such as a gene that includes a glowing, green fluorescent indicator - to be able to clear out every other non-heart cell in mouse models.

Earlier methods of performing genomic surgery had barriers of high costs and low flexibility that kept many researchers from adopting them.

"Then CRISPR started coming out, and since then it has absolutely exploded," said biologist Montserrat Anguera of Penn's School of Veterinary Medicine. "CRISPR seems to be the easiest and fastest way for labs to edit the genome."

She studies how embryonic stem cells develop into specialized cells within organs such as the liver or heart. Using CRISPR, she can delete regions of the stem cell genome to help decipher their function in human development.

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