REI Expedition MTS Stretch Union Suit
Here's what other sites are saying:
The REI Midweight MTS union suit is an excellent first layer for activities in cold weather. MTS(R) (Moisture Transport System) polyester with spandex is soft and stretchy for great comfort and fit; plus, it wicks moisture away from skin for comfort. Fabric provides UPF 50+ sun protection, shielding skin from harmful ultraviolet rays. Long front zipper runs diagonally for easy diaper changes. Features cuffed sleeves and legs; gusseted crotch creates a comfortable fit.