Mutants and symbiotes are potentially deadly enough beings in the Marvel Universe. Putting them together could potentially have disastrous results.
In X-Men Blue #21, it is just a single mutant who becomes attached to a symbiote, it's an entire team of X-Men.
The “Poison-X” story is a crossover between the X-Men and Venom that began in X-Men Blue Annual #1 and continues in X-Men Blue #21. Cyclops was conversing with his father, Corsair, the leader of the space pirates the Starjammers when the entire crew was attacked and captured by thugs wearing symbiotes. Cyclops couldn’t stand by while his father and Corsair’s crew were in danger, so he resolved to go to space and search for them. The other X-Men volunteered to go with Cyclops, but they realized they needed someone who was an expert in symbiotes to assist them. Who better than Venom?
The X-Men and Venom head into space. Naturally, they do what all spacefaring heroes do when they need information. They stop at a seedy bar and ask around. Word gets out that they’re searching for people with symbiotes and sure enough some symbiote-bonded thugs find them. A fight breaks out and the team gets away. Venom is able to forcibly extract some information from the informant who sold them out.
The informant points them to an arms dealer. Venom and the X-Men go to investigate, but things go poorly. A fight breaks out pretty quickly after the arms dealer is revealed to be selling symbiote and even implies that the Venom symbiote once belonged to him. The X-Men determine that they have ot free the symbiotes since they’re living things. After breaking the symbiotes out, the X-Men soon find themselves bonded.
That’s Cyclops, Angel, Beast, Marvel Girl, and Iceman all bonded to their own symbiotes. It's clear they’re still able to access their mutant powers as well, so it should be interesting to see their mutant gifts interact with the symbiotes’ abilities, but the question of whether they will control the symbiotes or the symbiotes will control them remains unanswered.
X-Men Blue #21 is on sale now. The “Poison-X” story continues next week in Venom #162.