Transcription factor required for the development of the heart and the spleen (PubMed:9584153, PubMed:19483677, PubMed:22560297). During heart development, acts as a transcriptional activator of NPPA/ANF in cooperation with GATA4 (PubMed:9584153). May cooperate with TBX2 to negatively modulate expression of NPPA/ANF in the atrioventricular canal (PubMed:12023302). Binds to the core DNA motif of NPPA promoter (PubMed:19483677). Together with PBX1, required for spleen development through a mechanism that involves CDKN2B repression (PubMed:22560297). Homodimer (via the homeobox); binds DNA as homodimer. Interacts (via the homeobox) with TBX5 (via the T-box); this complex binds DNA (By similarity). Interacts with HIPK1 and HIPK2, but not HIPK3 (PubMed:9748262). Interacts with the C-terminal zinc finger of GATA4 through its homeobox domain (PubMed:9584153). Also interacts with JARID2 which represses its ability to activate transcription of ANF (PubMed:15542826). Interacts with FBLIM1 (PubMed:14757752). Interacts with TBX18 (PubMed:17584735). Interacts with histone methyltransferase NSD2 (via HMG box) (PubMed:19483677). Predominantly in the adult and embryonic heart, and to a lesser extent in lingual muscle, spleen and stomach. Expression preceeds the onset of myogenic differentiation, and continues in cardiomyocytes of embryonic, fetal and adult hearts. It is also expressed laterally in future pharyngeal endoderm which is believed to produce the heart inducer. After foregut closure expression in endoderm is limited to the pharyngeal floor, dorsal to the developing heart tube. The thyroid primordium a derivative of the pharyngeal floor continues to express the protein after its levels diminish in the rest of the pharynx. The homeobox domain binds to double-stranded DNA. Belongs to the NK-2 homeobox family.