Hopping to the beat. Hop regulation of cardiac gene expression.

Hop is a recently discovered homeodomain only protein with atypical characteristics. Unlike other homeodomain proteins, Hop lacks DNA binding activity, although it localizes predominantly to the nucleus. During embryogenesis, Hop is expressed in the myocardium of the developing heart and it functions downstream of Nkx2-5, an early cardiac-specific transcription factor. One role of Hop is to modify the expression of cardiac-specific genes and thereby finely regulate heart development. This article reviews the discovery, characteristics, roles, and future perspectives of Hop.