The transgenic construct consisted of a sequence fragment encoding cre recombinase, a nuclear localization signal, and a consensus Kozak signal inserted at the start codon of the mouse genomic Nppa sequence. The 0.7 kbp Nppa proximal promoter region (-3kbp to +4 kbp relative to the transcription start site) was included. The mouse Nppa proximal promoter drives expression of cre recombinase in atrial cardiomyocytes beginning around E10.5. Complete expression is observed around E14.5. Some expression was observed in the inflow tract.