The PHD finger was discovered in 1993 as a Cys4-His-Cys3 motif in the plant homeodomain (hence PHD) proteins HAT3.1 in Arabidopsis and maize . The PHD zinc finger motif resembles the metal binding RING domain (Cys3-His-Cys4) and FYVE domain. It occurs as a single finger, but often in clusters of two or three, and it also occurs together with other domains, such as the chromodomain and the bromodomain.