Research involving human embryonic stem cells has sparked heated debate around the world, and is currently at the heart of a complex balancing act in the United States. On the one hand it is responsible for one of the largest US recruitment drives in recent years. But this surge in interest is tempered by politics and demarcated by state lines.

US law prohibits federal funds from being used on research that involves newly derived embryonic stem-cell lines. Private funds, however, do not suffer from such restrictions, and individual states are now trying to decide whether to plough their money into such an enterprise. Recent events in Missouri and Kansas, which both want to increase their national research presence, are cases in point.

In Missouri last month, university officials and biotech leaders had hoped that a state economic-development package would allow universities to combine private funds with public money to create endowed chairs. But this provision was quashed, in part because anti-abortion groups voiced concern that the money could be used to support embryonic stem-cell research.

Kansas faced a similar situation. Here, in April, the state passed a law to create a bioscience authority that would allow it to identify and recruit top researchers — but that basically bars embryonic stem-cell research.

The next battleground will be California, which already leads the country in terms of receiving funding from the National Institutes of Health. The California Stem Cell Research and Cures Initiative will provide $3 billion in stem-cell funding over ten years. So it will be up to California's voters in this November's referendum to tip the recruiting balance further towards their state by agreeing to more stem-cell funding, without restrictions. In the end, it may be up to researchers to follow the funding.