Reprogramming can cause cells to have an abnormal karyotype (particularly gains of chromosome 12 and 17), copy number variation, and point mutations, all tending toward amplification/overexpression of oncogenes and deletion/inactivation of tumor suppressors. At the epigenetic level, reprogrammed cells can retain a memory of the starting tissue from which they were derived. The cells can exhibit DNA methylation defects, particularly at CpG island shores, and aberrant histone modifications. They can also vary in X chromosome inactivation status.