Desmond-Hellmann’s departure from business’s upper echelons (She ranked No. 13 on Fortune‘s 2008 Power 50 list) adds to the trend of top women execs leaving corporations and deciding not to jump back in. Among the departed: former Procter & Gamble president Susan Arnold, former Pepsi-Cola North America CEO Dawn Hudson, former Yahoo president Sue Decker, and the trio who once were the most renowned women on Wall Street: Sallie Krawcheck of Citigroup , Zoe Cruz of Morgan Stanley , and Erin Callan of Lehman Brothers, whose recent leave from her new employer, Credit Suisse Group, is looking like it may be permanent.

All these onetime stars are on the sidelines except Hudson, who recently joined Parthenon Group, a Boston-based strategic advisory, as vice chairman — a three-day-a-week commitment to rachet down her stress level, Hudson says.

This decision by Desmond-Hellmann, 51, isn’t so surprising given Genentech’s fate: in March, Swiss drug giant Roche won a year-long battle to acquire the 44% of the biotech company that it didn’t already own for a whopping $46.8 billion. Chief executive Art Levinson, a Desmond-Hellmann fan who promoted her from clinical scientist to chief medical officer to EVP to president, lost the CEO title and remains chairman. Questions abound regarding whether Roche will be able to retain Genentech’s entrepreneurial culture. That culture has helped Genentech become not only the best company in biotech but also one of Fortune‘s Best Companies to Work For.

A onetime practicing oncologist who never imagined she’d climb the corporate ladder, Desmond-Hellmann is returning to her roots. She started her career at UCSF and, she says, “my heart has never left it.” She can’t talk at length about her move until the California Board of Regents approves her appointment. Stay tuned to Postcards next week to hear more from Desmond-Hellmann.

Meantime, have a great weekend!