If for example using Roche, partnered this year while still holding "accumulated" shares and then can buy shares as well, increasing their holding. With Genentech they partnered in the 90's then eventually bought out the remaining shares/buyout.
But Roche has owned a majority of Genentech since 1990,
Roche Holding’s agreement on Thursday to acquire full ownership of Genentech for $46.8 Billion (2009)