Transcription factor that plays an essential role in both trophoblast giant cell differentiation and in cardiac morphogenesis (By similarity). Binds the DNA sequence 5'-NRTCTG-3' (non-canonical E-box) (By similarity). Acts as a transcriptional repressor of SOX15 (By similarity). In the adult, could be required for ongoing expression of cardiac-specific genes (By similarity). Efficient DNA binding requires dimerization with another bHLH protein. Forms homodimers and heterodimers with TCF3 gene products E12 and E47, HAND2 and HEY1, HEY2 and HEYL (hairy-related transcription factors). Interacts with MDFIC (By similarity). Interacts with SOX15; the interaction enhances HAND1-induced differentiation of trophoblast giant cells (By similarity). Interaction with MDFIC sequesters it into the nucleolus, preventing the transcription factor activity. Phosphorylation by PLK4 disrupts the interaction with MDFIC and releases it from the nucleolus, leading to transcription factor activity (By similarity). Phosphorylation by PLK4 disrupts the interaction with MDFIC and leads to translocation into the nucleoplasm, allowing dimerization and transcription factor activity.