Genome stability of human embryonic stem cells (hESC) can be an essential concern because even minimal genetic alterations may negatively impact cell efficiency and safety. Rilpivirine nonhomologous end signing up for (NHEJ) fix may donate to their development. Inhibition of DNA-PK an integral NHEJ component by NU7026 led to a significant Rilpivirine reduction in radiation-induced chromatid exchanges in hESCs however not in somatic cells. On the other hand NU7026 treatment elevated the frequency of radiation-induced breaks to a similar extent in pluripotent and somatic cells. Thus DNA-PK dependent NHEJ efficiently participates in the elimination of radiation-induced chromatid breaks during the late G2 in both cell types and DNA-PK activity leads to a high level of misrejoining specifically in pluripotent cells. Keywords: human pluripotent cells DNA damage repair NHEJ chromosomal aberration G2 chromosomal radiosensitivity assay INTRODUCTION Pluripotent human embryonic stem cells (hESCs) are derived from the inner cell mass (ICM) of spare blastocysts and are able to differentiate into various cell types. Therefore these cells are often used as an in vitro model of the ICM. Recent studies suggest that a chromosomally aberrant cell populace is present in nearly all human spare embryos at the cleavage stage [1-3]. However newborns are characterized by a reduced frequency of chromosomal abnormalities when compared to preimplantation embryos [4]. In vivo the pluripotent cell state is maintained for a very limited time; however hESCs can be produced indefinitely in culture and their capacity to self renew and to differentiate into any cell type can be preserved for prolonged periods of time. These unique properties make hESCs very attractive being a potential way to obtain cells for healing usage. Obviously the genome balance of hESCs can be an essential issue to be looked at prior to make use of in scientific applications because also small genomic adjustments can considerably impair cell efficiency and safety. Many reports have supplied evidence of exceptional karyotype stability taken care of by some hESC lines during the period of a lot more than 140 -180 passages in vitro [5-6]. Nevertheless Rilpivirine Rilpivirine high-resolution karyotyping strategies have established that hESCs acquire chromosomal abnormalities during long-term passaging in vitro namely new sites of heterozygosity loss (LOH) and changes in copy-number variations (CNVs) [7 8 It is possible that this chromosomal aberrations observed in hESCs might reflect events much like those that occur in a developing embryo at the blastocyst stage. Later in development cells with normal karyotypes are selected by an unknown mechanism but hESCs accumulate chromosomal alterations during culturing in vitro. Repair Rilpivirine of DNA Flt4 double strand breaks (DSBs) by homologous recombination (HR) could be the source of the LOH arising in hESCs during cultivation while CNVs could potentially result from DSB repair by non-allelic homologous recombination (NAHR) non-homologous end joining (NHEJ) or microhomology-mediated end joining [9 10 A recent study aimed at characterizing DNA repair in hESCs indicates that HR is the major if not the sole mechanism of DSB repair in pluripotent human cells compared to differentiated somatic cells which typically use NHEJ [11]. However more recently Adams et al. [12] provided evidence demonstrating NHEJ functionality in hESCs and Rilpivirine showed that two closely-spaced DSBs induced by I-Sce endonuclease can be repaired with high fidelity by NHEJ in hESCs. NHEJ activity can result in chromosomal rearrangements when multiple DSBs coincide in space and time [13]. The aim of this study is usually to determine the repair accuracy of multiple radiation-induced DSBs in human pluripotent cells. To investigate the level of DSB misrejoining in pluripotent and somatic cells we used a G2-chromosomal radiosensitivity assay [14]. We analyzed radiation-induced chromosomal aberrations in solid-stained metaphases 2 hours following irradiation i.e. the cytogenetic analysis involved only cells irradiated during the late G2 stage of the cell cycle after transition through the G2/M checkpoint [15]. The design of this G2-assay allowed us to overcome the prominent differences in sensitivity to irradiation of pluripotent and somatic cells observed by Filion et al. [16] and.