XDx, a Brisbane, Calif. company that monitors the immune system with personalized medicine based on insights from the Human Genome Project, said it is raising up to $25 million more in cash.

The news, reported this morning by VentureWire (sub required) would be the company’s fifth round of venture capital. The company has already raised a total of $100.5 million since being founded in 2000. The fact that such a mature company, with well-known backers, is announcing it is raising funding suggests it may be having difficulty finding it, though we are not certain of that.

XDx is another one of those companies backed by venture capital firm Kleiner Perkins, which has made a big deal about the possibilities of personalized medicine in the wake of the Genome Project.

XDx says it has developed a molecular diagnostics technology that helps to non-invasively monitor the immune system by measuring gene expression in a patient’s peripheral blood. Monitoring the immune system is crucial in determining whether a patient can accept a transplanted heart, for example. XDx is already being used in 41 transplant centers.

Existing investors include Kleiner Perkins, DAG Ventures, Intel Capital, Texas Pacific Group Ventures, the Sprout Group (New Leaf Venture Partners), Burrill & Co., Integral Capital Partners and the Bay Area Equity Fund.

The company also appointed Mitchell J. Nelles, Ph.D., as vice president of research and development and technical operations.