UNDER ARMOUR. 1306428-036. MENS MK-1 SHORT SLEEVE SHIRT. STEEL LIGHT HEATHER WITH GRAPHITE. These are the go-to gym shirts for nearly every guy. The fabric dries fast to keep you cool and the streamlined fit cuts out any bulk or unnecessary material. It s also part of the UA MK-1 system--gear that all works together to give you what you need in any training situation. Fitted-- Next-to-skin without the squeeze. HeatGear fabric is ultra-soft and smooth for extreme comfort with very little weight. Mesh back and underarm panels deliver strategic ventilation. 4-way stretch construction moves better in every direction. Material wicks sweat and dries really fast. Anti-odor technology prevents the growth of odor-causing microbes. 90 percent Polyester with 10 percent Elastane.