SAN FRANCISCO—It's hard to imagine more divergent versions of the same business dispute than the ones laid out during opening arguments Monday by lawyers for Illumina Inc. and Ariosa Diagnostics—rivals in the burgeoning non-invasive prenatal testing (NIPT) industry.

To hear Illumina's lawyer, Edward Reines of Weil, Gotshal & Manges, tell it, Ariosa's “Harmony” test is a holdout in an industry that has by-and-large paid to license the Illumina gene sequencing technology. The technology allows physicians to use simple blood tests in place of a much more invasive procedure to screen for genetic defects.

“This ability to sequence quickly efficiently and accurately with the sequencing technology that Illumina developed opened up this whole NIPT area,” Reines said.

But David Gindler of Irell & Manella painted a starkly different picture for Ariosa—arguing that it did not infringe Illumina's two patents, and that the patents themselves were invalid. Gindler claimed that the whole dispute was cooked up by Illumina—an early Ariosa investor—in efforts to sidetrack Ariosa's efforts to raise funds via an initial public offering.

“We have our technology. They have theirs,” Gindler said. “They did this for strategic business reasons,” Gindler said.

The adversaries will be squaring off in a high-stakes trial that's set to run over the next three weeks in U.S. District Judge Susan Illston's San Francisco courtroom. Illumina is set to seek as much as $300 million should the jury find that Ariosa infringed willfully. Ariosa, in turn, is seeking tens of millions of dollars it claims Illumina owes for breaching its contract to provide Ariosa's testing supplies—and for suing in bad faith.

The jury of six men and two women who will decide the case was seated before lunch on Monday and the parties moved to opening arguments in the afternoon,

Illumina's Reines, who opened first, said that his clients had developed “revolutionary new improvement in maternal health care,” which all the major providers of NIPT services besides Ariosa had paid to license. Reines added that the “big twist” in the dispute is that two of Ariosa's executives, including co-founder John Stuelpnagel, served as the named inventors of one of the two patents in the suit during prior stints working at Illumina.

That fact, Reines said, put the Ariosa executives in the ideal position to recognize that the “homebrew” method they developed to perform the tests infringed an Illumina patent. “They wanted to be the low-cost provider [after] being a little late to the market,” Reines said.

Gindler seized on the remarks about Ariosa's Stuelpnagel, noting that Reines had neglected to inform jurors that Stuelpnagel actually co-founded Illumina. Gindler said that Stuelpnagel, after moving on to Ariosa, specifically referenced the patent he had worked on in presentations to investors and indicated that it wouldn't be an issue with his new company's technology.

Gindler argued that Illumina's lawsuit was foreclosed by a commercial agreement that gave Ariosa license to some of the underlying technology, and said the company is seeking at least $17.5 million for breach of contract under that agreement. Following Illumina's suit, Ariosa redesigned its Harmony test to run on an DNA analysis platform supplied by another company, Affymetrix.

“All that we want is our money back,” Gindler said.

Correction: An earlier version of this story misquoted Illumina's lawyer saying the company developed ”revolutionary new improvement in paternal healthcare.” The quote has been corrected to say “maternal healthcare.”