Cattlemen in the Old West seared the names of their ranches into their animals' hides to discourage rustlers and help lawmen identify stolen cattle.

Today's law enforcement officers, however, have a much more sophisticated tool - DNA typing - to help stem the age-old crime.GenMark Inc., a Salt Lake laboratory that specializes in agricultural DNA typing, recently prepared evidence that helped a Florida prosecutor convict a cattle rustler. It may be one of the first rustling convictions in the country obtained using genetic evidence.

DNA, or deoxyribonucleic acid, is the genetic blueprint of life. Each animal, including man, has its own unique DNA patterns in its blood and tissue cells.

In the Florida case, undercover wildlife officers posing as meat buyers agreed last summer to purchase from a suspect 92 pounds of beef they believed was from a stolen, butchered cow.

By fall, officers located a rancher they theorized was the stolen cow's owner. They also believed the dead cow had been sired by a bull in the rancher's herd.

GenMark compared samples of beef and the blood of bulls owned by the farmer. The tests revealed that the meat likely was sired by a bull in the farmer's herd.

The defendant pleaded no contest to dealing in stolen property and was sentenced to prison for 15 years for that and other crimes.

Preparing evidence for a criminal case was a little bit out of the ordinary for 2-year-old Gen-Mark, which was the first company in the world to offer DNA typing to cattle organizations.The tests determine the cattle's parentage, which has become increasingly important because most breeding is conducted through artificial insemination and embryo transfers.

"The breed associations want the animals to be registered. What they really want to do is confirm the parentage of all the new offspring," said Tom Holm, GenMark's senior production manager.

DNA typing has become an increasingly popular tool in criminal investigations, linking suspects to crime scenes by the root of a hair, a drop of blood or a tiny piece of skin. "One of the main advantages is it works on almost any cell in the body," he said.

Yet, Holm is careful not to overemphasize the importance of DNA typing as criminal evidence. "Really, it just provides additional information. It's just a supporting piece of evidence in a criminal investigation," he said.

In proving paternity cases, however, Holm said he expects that DNA typing will render conventional blood tests obsolete. Gene typing tests carry a 99.0 percent or greater probability of parentage.

"It's coming to the point that state legislatures are recognizing the power of DNA typing and demanding it is the primary technology used in determining paternity," he said.

Holm predicts that the genetic tests eventually will be commonplace. The lab's greatest challenge is do develop technology for widespread use.

DNA was discovered in the sperm of Rhine River trout in 1871. But many of the gene typing techniques employed by GenMark are only a decade old and were developed by University of Utah researcher Ray White.

White was among a team of researchers who earlier in March reported the discovery of a gene suspected of playing a key role in colon cancer's early development. Last summer, White and other researchers identified and cloned a gene that causes neurofibromatosis type 1 - a disease that can cause blindness from tumors in the nervous system and the eye, kidney disorders and occasionally mental retardation.

Another segment of GenMark Inc.'s business is cloning cattle embyros. The company has developed the technology to remove an embyro from the womb five days after conception, split and clone it. The cloned embryo could then be sold to the rancher or another interested party.

Clones would be marketed according to the presence of genes which indicate the cow would produce a lot of milk, produce milk better suited for cheese making or genes that indicate the cattle is resistant to certain diseases - all identified through DNA typing.

"We're not putting new genes into cattle," Holm emphasized. Instead, the company is helping determine what will be the breeding stock for the next generation of cattle.

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(Chart)

- A lab technician isolates DNA from a sample, usually a blood sample.

- The DNA is cut in specific pieces using a restriction enzyme.

- A portion of the DNA sample is loaded into channels cut in a solid support gel.

- Electrical current forces the DNA pieces to migrate into the gel and separate according to size.

- The gel is then blotted with paper towels and transferred to a nylon membrane.

- A radioactive DNA probe and the membrane are mixed together.

- Identical DNA molecules are attracted to one other and bind to the nylon membrane.

- The nylon is exposed to a film, from which the dark, identifying bands are visualized.

- The dark bands reveal differences between individuals.