Stem cells are promise and threat at the same time. To understand how stem cells act it is crucial to study their behavior in their natural context. Following and manipulating an individual stem cell and its descendants on their path to differentiation has so far not been possible. We have recently developed new microscopy and image analysis tools that allow long term in vivo observations. Novel genetic tools allow to stochastically label individual cells and thus to follow all descendants derived from the cell initially marked. We have modified and adapted this technology for the fish system and will extend its potential to allow functional clonal analysis in vivo. The transparent fish medaka (Oryzias latipes) and zebrafish (Danio rerio) are ideally suited to combine advanced imaging and the genetic lineage manipulation of stem cells. This way the influence of genetic pathways implicated in cell proliferation and differentiation is directly addressed in vivo. Clones originating from single wild type or manipulated cells are followed in their in vivo context and the physiological consequences of this manipulation is directly measured. Individually labelled clones will be followed by DSLM 4D microscopy over periods of up to 7 days in juvenile and adult fish. We will interfere with signaling pathways and key transcription factors in these cells combining the brainbow approach with lineage specific gain- and loss-of-function. That way a specific color will indicate a specific experimental condition. We will focus on the path of decisions taken by retinal stem cells and their descendants on the one hand and on cells composing induced tumors on the other. This will be highly relevant for our understanding of stem cell and tumor cell proliferation and differentiation.