A genetic engineer is a monster in SLASH'EM. Its attacks cause monsters or you to polymorph, making it both a formidable opponent and a useful polymorph form. This attack is nullified by magic resistance, and affected by magic cancellation.
The following information pertains to an upcoming version (NetHack 3.7.0). If this version is now released, please verify that it is still accurate, then update the page to incorporate this information.
Genetic engineers have been introduced to NetHack 3.7. They can appear outside of Gehennom, and can be summoned with summon nasties. They cannot teleport. Eating a genetic engineer corpse will cause you (or a monster) to polymorph, as a chameleon corpse does; magic resistance does not nullify this effect. Currently, pets that are poison resistant will not avoid eating them.
If you are a werecreature, the polymorph attack will instead switch you between your natural and beast forms.
Living with genetic engineers
Hopefully by the time you reach Gehennom in SLASH'EM, you will have magic resistance, making genetic engineers almost a non-threat. If you don't, then ranged weapons are advised; even though their attacks are affected by magic cancellation, they have a reasonable chance of being able to polymorph you once; if they then polymorph you into something that destroys your cloak and armor or causes you to shrink out of it, all their subsequent attacks will polymorph you as well, as you will no longer have any magic cancellation.
Also be careful of them if you are causing conflict. Since monsters in SLASH'EM are not killed when you destroy their polymorphed forms, but rather revert to their original form, at best a genetic engineer hitting a number of monsters causes extra fighting for you. At worst it creates a crystal golem, which is not only almost certainly stronger but also resistant to further polymorph, in the manner of a gray dragon. If you are causing conflict, destroy genetic engineers with prejudice.
Living as a genetic engineer
Being a genetic engineer is quite an interesting life indeed. The polymorph attack is not stopped by monster MR, meaning that except against aforesaid crystal golems, (baby) gray dragons and any monster that has armor which provides magic resistance or magic cancellation, your attack will polymorph them. This can be quite powerful, as it allows you to bypass or tame numerous formidable enemies, including some that cannot normally be tamed, or even monsters that are simply very difficult to tame. This is in fact the only way to tame Death in SLASH'EM, not counting sliming him which makes him permanently a green slime. However, shopkeepers, aligned priests, and hostile minions can never be tamed, even if polymorphed.
Being a genetic engineer does have its limitations. The form is not very strong, and many monsters may destroy you before you get a chance to polymorph them, particularly if you are surrounded. Also, there is the ever-present risk of creating a stronger monster (including the un-polymorphable crystal golem). Also be aware that if the attack does not pass the "needs +x weapon to hit" check, it will not polymorph a monster. The bare-handed attack counts as +0, but wielding a weapon will make it count as the enchantment of the weapon. Unlike demonic summoning, the genetic engineer's polymorph attack is not affected by whether or not you are wielding a weapon; it will polymorph monsters regardless.
Beware that using the polymorphing attack will trigger passive attacks from the monster's new form rather than the old. Attacking without gloves can lead to YASD if a Footrice, Asphynx or Basilisk is created.