Differential expression of different genes at different stages of development leads to organ and tissues differentiation, for example, size and pattern of puffs of polytene chromosomes, which are site of transcription, varies at different developmental stages, indicating differential gene activity. Transcription factor is the molecular basis of organ differentiation. Related Theory The fate of a cell describes what it will become in the course of normal development. The fate of a particular cell can be discovered by labelling that cell and observing what structures it becomes a part of. The developmental potential, or potency, of a cell describes the range of different cell types it can become. The zygote and its very early descendants are totipotent - these cells have the potential to develop into a complete organism. The determination of different cell types (cell fates) involves progressive restrictions in their developmental potentials. Differentiation follows determination, as the cell elaborates a cell-specific developmental program. Differentiation results in the presence of cell types that have clear-cut identities, such as muscle cells, nerve cells, and skin cells.