You can't patent (naturally occurring) genes

In an unanimous decision, the United States Supreme Court ruled today that companies can't patent genes, or parts of genes — at least, so long as that genetic material is identical to what occurs in nature. The lawsuit dealt specifically with Myriad Genetics, the company that isolated and has claimed a patent on BRCA 1 and BRCA 2 — genes associated with an increased risk of breast and ovarian cancers. From a practical perspective, Myriad's hold on the genes has meant that tests for genetic cancer risk are strikingly expensive — Xeni paid more than $3000 for hers. It's also meant that, if you get a positive result, there's been nowhere you could go for a second opinion.That's a big deal. Mutations in the BRCA 1 and 2 genes mean an increased risk of cancer, but there's more than one kind of mutation that can happen. In fact, BRCA 1, alone, has hundreds of known mutations. Some increase your risk of cancer. But, even if you narrow it down to just those, they don't all increase the risk by the same amount. The health choices you make could be very different depending on whether you have an 80% risk of developing breast cancer by age 90 (the worst-case scenario for BRCA 1 mutations), or something much lower. That's the kind of situation where you might really like to have more than one lab run more than one kind of test.
This ruling opens the door for that, and the competition should (theoretically) also lower the cost.
It's worth noting that the ruling does not apply to synthetically created DNA, which, based on the Myriad context, seems to apply to lab-created genes that are different from what happens in nature, as opposed to lab-created versions of natural genes (Myriad had previously cloned BRCA 1 and BRCA 2). For a little more clarity on what does not fall under the scope of this ruling, here's a quote from Justice Clarence Thomas' description of the court's decision. It's the cDNA Thomas describes here that would still be patentable.

DNA’s informational sequences and the processes that create mRNA, amino acids, and proteins occur naturally within cells. Scientists can, however, extract DNA from cells using well known laboratory methods. These methods allow scientists to isolate specific segments of DNA—for instance, a particular gene or part of a gene—which can then be further studied, manipulated, or used. It is also possible to create DNA synthetically through processes similarly well known in the field of genetics. One such method begins with an mRNA molecule and uses the natural bonding properties of nucleotides to create a new, synthetic DNA molecule. The result is the inverse of the mRNA’s inverse image of the original DNA, with one important distinction: Because the natural creation of mRNA involves splicing that removes introns, the synthetic DNA created from mRNA also contains only the exon sequences. This synthetic DNA created in the laboratory from mRNA is known as complementary DNA (cDNA).

Read More:
• The Washington Post on the Court decision
• The full Supreme Court decision itself
• Rebecca Skloot, who literally wrote the book on the ethics of genes and genes patents, is answering questions at Facebook.