HH21

HH21, HH-21, HH 21, HH.21, may refer to:


 * Piasecki HH-21, a variant of the Piasecki H-21 helicopter


 * HH21, one of the Hamburger–Hamilton stages in chick development
 * FLRT3, leucine-rich repeat transmembrane protein 3, also called "HH21"