Updated from 1:32 p.m. EST

Shares of biotechnology highflier

Genentech

(DNA)

dropped 11% on Thursday after its parent company,

Roche Holding

of Switzerland, said it intended to sell off 19 million shares, reducing its stake to 58% from 66%.

Daniel Piller, a spokesman for Roche in Basel, said the company decided to sell the stock in order to "enhance our financial position and create bigger liquidity for Genentech."

Nineteen million shares would be worth $3.99 billion based on Wednesday's closing price of 209 3/4.

But that value had eroded somewhat, as Genentech dropped 21 1/2, or 10%, to close at 188 1/4. Genentech has steadily risen from 58 1/4 when it began trading in July.

Piller said the company's plans were outlined in a filing with the

Securities and Exchange Commission

. Roche gave no timetable for selling the shares.

Piller noted, however, that the relationship with Genentech would not change. He declined to comment on what Roche intended to do with the money it raised from the sale of Genentech shares.

Roche bought all of the outstanding shares of the South San Francisco, Calif.-based last year by exercising options it obtained in 1990 and 1995. The company then took Genentech public again in two stock offerings last year.