Project Sign, just like project Grudge, is an earlier version of Project Blue Book.
Project Sign was the first project to officially investigate UFOs. It was short-lived: it was started in 1948 and already by the end of 1948 it was succeeded by Project Grudge. The main reason for this is that by fall 1948 the project Sign team had come to the conclusion that UFOs were most likely extraterrestrial - or 'interplanetary' as they called them - ships. They had written this in the so-called "Estimate of the Situation." When this paper reached the Pentagon, the staff of Project Sign were told that their conclusions were unacceptable and General Hoyt Vandenberg ordered that all copies of the Estimate had to be destroyed. The project was broken up and replaced by project Grudge.