Among mobile DNA elements, Long interspersed element-1 (LINE-1 or L1) is the only autonomously active human retrotransposon. L1 mobilises from one genomic location to another via a copy-and-paste mechanism, known as retrotransposition. The L1 sequence contains an internal promoter that drives the expression of an mRNA encoding 2 proteins: ORF1p, with nucleic acid chaperone activity and ORF2p, with endonuclease and reverse transcriptase activities. Truncations and point mutations have rendered most of L1 copies immobile, but approximately 100 copies in humans are still mobile. Over the past decades, L1 mobility was identified in several contexts, such as neural precursors and mature neurons, germ cells, and cancer. However, its role in normal cell physiology and disease remains unclear. This project aims to modulate L1 activity in vitro and study its consequences on various biological processes, such as cell survival, proliferation and differentiation. We have generated an inducible L1 reporter cassette (iL1-EGFP) consisting of a full length human L1 with ORFs tagged for better protein detection, and an antisense EGFP cassette that activates only after a successful mobilisation event. Here, iL1-EGFP activity is controlled by viral delivery of CRE recombinase. We will use this system to test the efficiency of retrotransposition and consequences of L1 upregulation in cell lines, such as HeLa, HEK293T and SHY5Y. Additionally, we will use a CRE conditioned-CRISPR activation system targeting the L1 native promoter to study the effects of endogenous L1 transcriptional activation. Contrarily, we will downregulate L1 expression in a cell-type specific manner by using CRE-dependent short hairpin RNAs and CRISPR interference. This project will generate novel tools to modulate L1 expression and mobility, providing insights into the consequences of altered L1 activity on cell biology.