An ideal lightweight baselayer under your fleece and softshell, or worn alone in warmer weather, the Mountain Hardwear Advance Lightweight Short Sleeve Crew for Boys is made of Advance Lightweight fabric, a 100 polyester which wicks moisture, dries quickly and is antimicrobial to keep you feeling dry and nice even during high output aerobic activities.More like a second skin that a shirt, the Advance Crew moves with you and thanks to flat lock seams and strategic seam placement ensure you won't get rubbed the wrong way during an epic run or long day in the backcountry.