Targeted Genetics stock fell more than 20 percent today after the company interrupted clinical testing of its lead arthritis drug because of a patient's illness.

As of 2:24 pm Eastern time, the Seattle-based company's shares traded at $2.07, down 56 cents or 21.29 percent.

The company said late Tuesday that it had stopped trials of its tgAAC94 therapy after a patient became seriously ill. It's unclear whether the adverse effects are related to the drug.