The Marvel Universe is the shared fictional universe in which occur most events published by Marvel Comics. However one of the main exceptions are the G.I. Joe comics, which notably take place in a universe where the regular Marvel characters are absent.
Such an arrangement is normally rare, although the other major Hasbro/Marvel comic series, Transformers, also does not take place in the Marvel Universe (albeit after a few early indications to the contrary). Marvel comic adaptations of other toys such as the Micronauts and Rom were unambiguously set in the mainstream Marvel universe and often featured guest appearances and crossovers with the regular Marvel characters. However G.I. Joe aimed for a much greater degree of realism and thus set aside the prospects of crossovers. Its status apart was confirmed in letters pages and also by the lack of any crossover with the Marvel Universe-wide event Secret Wars II.
Nevertheless there have been a few occasions when characters from the Marvel Universe have overlapped on the world of G.I. Joe, though the wider ramifications have been ignored.
Perhaps the most prominent example came in the UK comic Action Force when issue #17 ran the story "Meditations in Red" which introduced the Marvel Universe character of Shang-Chi, the Master of Kung Fu, together with several of his supporting cast and his arch enemy Fu Manchu, as a prelude to the beginning of reprints from the 1970s Marvel US series Master of Kung Fu. The story revealed that Shang-Chi had trained Quick Kick in martial arts, thus explicitly linking the two together. Reference was also made to other Marvel Universe characters such as Iron Fist, Batroc and Elektra.