With the advent of high throughput RNA sequencing technology and higher density tiling arrays, the transcriptional activities across the human genome have been investigated and led to the observation of widespread transcription of the genome . While only less than 2% of the mammalian genome codes for proteins, tens of thousands of genomic sites are pervasively transcribed and produce non-coding RNAs, including microRNAs, Piwi-interacting RNAs, small interfering RNAs, small nucleolar RNAs and long non-coding RNAs (lncRNAs). LncRNAs were originally defined as transcribed RNA molecules longer than 200 nucleotides and with little protein-coding potential. Recently, emerging evidences have shown that lncRNAs play important roles in a variety of biological processes including development, differentiation, metabolism, genome regulation and cancer progression . Nonetheless, due to the heterogeneity and low abundance, in most cases, most lncRNAs have no genetic evidence to support their in vivo function. Gene-targeted knockout technology has provided a powerful tool for elucidating the function of lncRNA genes in vivo, making the connection from mouse to mechanism. For example, Malat1, known as metastasis associated lung adenocarcinoma transcript 1, is among the most abundant and highly conserved lncRNAs. Malat1 was identified as an oncogene that promoted tumorigenesis and found to regulate pre-mRNA splicing in nuclear speckles and promote E2F1 target gene expression during cell cycle progression in vitro . Recently, Malat1 knockout models have shown that loss of Malat1 in vivo has no apparent phenotypes and compatible with formation of nuclear speckles , but alters the transcription of Malat1 neighboring genes and metastasis-associated genes . These loss-of-function models further support the critical function of Malat1 as a regulator of gene expression governing hallmarks of lung cancer metastasis. Along with more and more differentially expressed lncRNAs have been unraveled and screened during various physiological processes and disease, understanding the functions of these lncRNA genes require lose-of-function screen by deleting or modifying genes, and followed by studying the resulting phenotypes. More recently, a novel powerful genome-editing tool - the type II prokaryotic CRISPR/Cas system has been successfully employed for genome engineering, thus holding great potential for deciphering the function of lncRNAs in vivo.

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