ImClone Systems ( IMCL) shares rose about 8% Thursday following news late Wednesday that Carl Icahn had accumulated a 6.9% stake in the biotech company. Last month ImClone received long-awaited approval from the Food and Drug Administration for its Erbitux colon cancer treatment. Erbitux's FDA approval process has a storied past. The company's former co-founder and CEO Samuel Waksal is now serving a prison sentence for trying to dump ImClone shares in 2001, ahead of the announcement of an unfavorable FDA ruling on the drug. Home fashion diva Martha Stewart, a friend of Waksal and founder of Martha Stewart Living ( MSO), is now on trial for obstruction of justice charges relating to the circumstances under which she sold shares of ImClone shares around the same time. According to documents filed with the Securities and Exchange Commission, Icahn holds some 5.2 million shares of ImClone. ImClone shares rose $3.47, or 7.9%, to $47.35, near their 52-week high of $48.93. Icahn told Reuters that he thought Wall Street had underestimated the "great potential" of Erbitux.