defines a subpopulation of pacemaker cells and is essential for the physiological function of the sinoatrial node in mice.

The sinoatrial node (SAN), the primary cardiac pacemaker, consists of a head domain and a junction/tail domain that exhibit different functional properties. However, the underlying molecular mechanism defining these two pacemaker domains remains elusive. Nkx2-5 is a key transcription factor essential for the formation of the working myocardium, but it was generally thought to be detrimental to SAN development. However, is expressed in the developing SAN junction, suggesting a role for in SAN junction development and function. In this study, we present unambiguous evidence that SAN junction cells exhibit unique action potential configurations intermediate to those manifested by the SAN head and the surrounding atrial cells, suggesting a specific role for the junction cells in impulse generation and in SAN-atrial exit conduction. Single-cell RNA-seq analyses support this concept. Although inactivation in the SAN junction did not cause a malformed SAN at birth, the mutant mice manifested sinus node dysfunction. Thus, defines a population of pacemaker cells in the transitional zone. Despite being dispensable for SAN morphogenesis during embryogenesis, its deletion hampers atrial activation by the pacemaker.