Spark constructs consist of two coding regions that are separated by a 2A linker (resulting in separate translation of each protein). The first coding sequence expresses a consensus peptide that is phosphorylated by a specific kinase, the EGFP fluorescent protein and 'HOTag3' (a homo-oligomeric coiled-coil sequence that forms hexamers). The second coding sequence consists of a phospho-peptide binding domain fused to 'HOTag6' (a homo-oligomeric coiled-coil sequence that forms tetramers). Upon activation of the kinase, the consensus peptide is phosphorylated and interacts with the phospho-peptide-binding domain. The presence of the 'HOTag3' and 'HOTag6' homo-oligomeric coiled coils on the respective proteins results in multivalent protein-protein interaction, leading to phase-separation into intensively fluorescent droplets.