From Reason:

An initiative mandating that foods containing genetically modified organisms carry warning labels has made it onto the ballot in California. Proposition 37, the California Right to Know Genetically Engineered Food Act initiative, is an anti-science campaign that flies in the face of an overwhelming scientific consensus that genetically modified foods are safe and healthy. Corporate sponsors are working closely with unaccountable special interest groups in a disinformation campaign designed to frighten and confuse voters.

The Proposition 37 petition asserts that “genetic engineering of plants and animals often causes unintended consequences. Manipulating genes and inserting them into organisms is an imprecise process. The results are not always predictable or controllable, and they can lead to adverse health or environmental consequences.” All of these claims, quoted from the findings and declarations section of the initiative, are solidly contradicted by the scientific consensus regarding biotech crops.

For more, see: California Initiative Puts Profit Ahead of Science