William Blair analyst Andy Hsieh recommends being an "aggressive" buyer of Seattle Genetics (SGEN) shares on today's post-earnings selloff. The stock in premarket trading is down 18%, or $11.93, to $53.42. The weaker than expected Q3 earnings were driven by a combination of lower than expected Adcetris sales, increased R&D spending, and the recognition of a loss of $21.9M in investment associated with owning Immunomedics (IMMU) shares, Hsieh tells investors in a post-earnings research note. With investors "keenly focused" on the company's progress in launching Adcetris in the frontline Hodgkin lymphoma setting, the fact that Adcetris sales of $127M failed to meet management guidance of $130M-$135M will be viewed as a disappointment, adds the analyst. However, he believes the setback is "temporary and the situation a function of challenging patient flow dynamics." Hsieh says his long-term bullish view on the Adcetris franchise remains unchanged. He keeps an Outperform rating on Seattle Genetics.