The aliens in the Second Alien War are controlled by small bio-mechanical implants in their bodies, allowing an unknown force to exert control over them as if the alien were simply an extension of this force's body. While X-Com does Psionic screening of all troops, this molecular control technology is different, and so a new method of combating it must be developed.
This is where the M.C. Lab comes into play. Doubling as a surgical facility and training center, soldiers assigned to M.C. Lab training are surgically implanted with devices mirroring those of the aliens. There is no downside to this, as the aliens can somehow exert Molecular Control even over unimplanted soldiers. Different soldiers will more readily accept the implant than others, leading to varied levels of M.C Strength, depending how well the implant was accepted.
The M.C. Lab serves as a training facility for implanted soldiers, where they can utilize a complex computer system and implanted lab animals to develop their M.C Skill. The nice people at Alien Containment may even bring down a more accurate testing subject every now and then. One M.C. Lab can train implant and train ten soldiers. At the end of the first month of training, soldiers without any M.C. stats will now show them, and they will have 16-24 M.C. Skill points.
Soldiers who already had some M.C. training will continue to increase their Skill. Just as importantly when you first train any recruit you will learn their M.C. Strength which is a statistic you cannot improve with training, above 80 is good, above 95 is excellent. Note that your M.C. Labs will continue to train your soldiers, even once they reach the combat cap of 100.
Many of the same principles used in the first alien war's psionic training apply as well to M.C. training. Therefore, ror more specifics on M.C. Lab training (and to avoid repetition), see Psi Skill - Improvement.
Try not to transfer soldiers if they are using the M.C. Lab. Training for that month will be lost, even if the destination base has M.C. Lab space available. You can use soldiers in combat whilst they are training, but if they die, you'll have an empty slot in your labs, which can't be filled until the following month.
Soldiers able to use M.C. Disruptors due to training in the M.C. Laboratory are a powerful asset, but this is countered somewhat due to the restrictive method by which this training is obtained.
Note that you cannot dismantle any M.C. Labs at a base if even just one soldier is in training. So place them carefully, planning for future growth such as Sub Pens, etc. Or edit your base with something like XcomUtil.
Be sure to have an M.C. Reader in storage when you complete research on the M.C. Lab, otherwise you will not ever be able to learn the secrets of reading the alien's implants, and thus never be able to use the M.C. Lab for anything more than screening.
Construction Time: 24 days
Construction Cost: $750,000
Maintenance Cost: $16,000/month