Zombotech Corporation is a large, semi-enterable office tower in Downtown, San Fierro which is currently engrossed in zombie virus research.
The lobby is housed in a small beaux arts atrium that attaches to a modern tower. The inside features a sculpture of a DNA strand, possibly hinting that human DNA is being altered to make zombies. The tower itself has an open air cut out in the center of its core to allow sunlight in. At the back of the building are elevators that go to the underground labs for zombie research.
One of the signs in the building proves the corporation is actually working on a zombie virus, stating: "Zombotech Sinister Zombie Virus Research Corporation Welcomes Visitors!". Whether zombies actually exist within the game is debated, although some pedestrians in the game resemble them.
About halfway up the DNA strand is a snapshot collectible, possibly contributing to speculation about the building.
Inside the building, there is a diagram of the Zombotech Corporation building. It clearly indicates that there is an underground bunker beneath the building. The picture just might be here to give the impression that the building is bigger than it looks, and that there might be secret tests or laboratories inside the bunker. However, there is only one known bunker in the game, but it's Area 69.
If the player enters Blue Hell and flies under the building, there will be nothing there. Technically this proves very little as interiors are generally placed much further from the building they are supposedly inside, but the elevator does not seem to be animated or any more than decoration, so it is unlikely there is any more than the first floor. Furthermore, there is no interior that exists within the Interiors Universe that looks anything like a secret laboratory.
Easter Egg Connections
The building is very likely a reference to the Umbrella Corporation from the Resident Evil series, simply poking fun towards the company known only for zombies and multistory bases.