The human genome is largely composed of non-coding DNA, only 3-5% codes for exons. The majority of the non-coding DNA is however fundamental for the correct functioning and regulation of the genome. Transposable elements (TEs) like LINEs generated up to 50% of the human genome during evolution. They can mobilize, causing mutations but also conferring genomic plasticity. The generation of new insertions in the germ line led to the concept of TEs as ‘selfish’ DNA. However, inconsistent with their hereditary transmission, it was recently shown by the host lab and others that most of the action of TEs occurs in somatic cells during early embryogenesis.Thus, to deeply understand the impact of the somatic activity of LINE elements, we propose to develop and use an in vivo mouse model for LINE transposition. We will use human embryonic stem cells (hESCs) and tissue-specific iPSCs, containing an eGFP-marked LINE reporter cassette. Once injected into immuno-suppressed mice these cells will develop teratomas containing tissues of the 3 germ layers (endo, meso, ectoderm). For the proof of principle, the host lab has injected human embryonic carcinoma cells (hECs). With this model, we aim to answer the following: 1) Are LINEs active in all three germ layers?; 2) Are LINEs differentially regulated, depending on the germ layer?; 3) What’s the impact of LINE activity in the somatic tissues?To answer these questions, we will FACS-sort parts of the teratoma into ecto-, meso-, and endoderm-like cells and map the site of new LINE insertions during development, using deep sequencing. We aim to detect hot spots of LINE integration, allowing the analysis of possible genetic and phenotypic consequences. Imaging of the rest of the teratoma will further show us cell types where transposition occurs with higher frequency and possible phenotypic alterations. We therefore hope to detect new principles of genomic plasticity that could regulate gene expression.