Streptophyte algae are either unicellular or form multicellular filaments, branched or unbranched. Life developed from prokaryotes into eukaryotes and multicellular forms. Additionally, interspecies signaling occurs between multicellular organisms. Tetraspanins are a family of membrane proteins found in all multicellular eukaryotes. Yet sexual reproduction is the more common means of reproduction among eukaryotes and multicellular organisms. Apoptosis is a form of programmed cell death in multicellular organisms. Cells within colonies became increasingly specialized, resulting in true multicellular organisms. Some protozoans can form multicellular colonies. Many transcription factors in multicellular organisms are involved in development. Cell signaling coordinates cellular activities, and hence governs the basic functions of multicellular organisms. The signaling molecules used by multicellular organisms are often called pheromones. The clearance of early apoptotic cells is an important function in multicellular organisms. PRC2 are present in most multicellular organisms. Gametes, for example, are reproductive unicells for multicellular organisms. The zygote divides by mitotic division and grows into a multicellular, diploid sporophyte.