Columbia Women's Midweight II L/S Half Zip T-Shirt 100% Polyester Zoned thermal-reflective technology at front and back torso and breathable, sweat-wicking side inserts give you what you need, where you need it, while a versatile midweight fabric lets you wear this comfy long sleeve baselayer top in a wide variety of conditions and situations Flat-lock seams slide smoothly over your skin, underarm gussets deliver great mobility, and a half-zip collar allows you to customize your ventilation