Uni-Ball Signo Extra Fine Point Gel Pen Refills, Red Ink, 0.28 mm, Set of 6 (Japan import) [Komainu-Dou Original Package] Refill for Uni-Ball Signo UM-151-28.15 Extra Fine Point Gel Ink Pen Ink color: Red The diameter of a ball: 0.28mm. Extra Fine Point.