Dear Editor,The ubiquitously expressed CCCTC-binding factor (CTCF),is highly conserved from Drosophila to mammals and plays multiple functions in the genome (Ohlsson et al.,2001).CTCF has been shown to establish chromatin insulation in vertebrate,and it also plays the roles in transcriptional regulation,X-chromosome inactivation,and imprinting of genes (Phillips and Corces,2009).In addition,CTCF plays a pivotal role in genomic organization and loop formation by mediating long-range chromatin interactions between distant loci (Yao et al.,2010;Tang et al.,2015).Several hypotheses have been proposed to explain the diverse functions of CTCF.The popular ‘zinc-finger model’ proposed that the CTCFs different functions are due to the interplay between the zinc-finger engagement and the underlying sequence differences (Ohlsson et al.,2001).Genome-wide studies have identified that the majority of CTCF binding sites belongs to a set of nonpalindromic CTCF binding sites with a consensus sequence referred to as M1 (Kim et al.,2007;Schmidt et al.,2012).Recently,another binding motif,referred to as M2 and 5-6 bp upstream of M1,has been discovered (Schmidt et al.,2012).Moreover,CTCF zinc fingers (ZFs) 4-8 strongly bind to the M1,while ZFs 7-11 tend to strongly bind to the M2 (Renda et al.,2007;Xiao et al.,2015).In this study,we aim to compare the binding abilities of CTCF to M1 and M2 and determine which bases were requirement for M1 and M2 bind to CTCF.