Smartwool. Our Merino 150 Base Layer Short Sleeve is all about year-round comfort and performance—so you can continue to get after it in your soon-to-be favorite shirt. With our Core Spun technology, we've wrapped Merino around a nylon core for added durability while still keeping the comfort of ultra-soft, responsibly sourced Merino next to skin. And select colorways feature a more responsible plant-based-dye technique. Built for consistent comfort, even when wearing a pack, this shirt is ready to explore the world on your back.