Table 1.

Epigenetic barriers to somatic cell nuclear transfer cloning.

Epigenetic Barrier	Where	Reference
Memory of an active transcriptional state	Donor cells	[55,56]
Imprinting disorder in donor cells	Fibroblasts from abnormal cloned fetuses	[57]
Misregulation of mRNAs at the time of ZGA	Early stage NT embryos	[35]
Disturbed transcription by RNA polymerase I around ZGA	Early stage NT embryos	[38]
Non-proper degradation of maternally stored transcripts	Early stage NT embryos	[35]
Continuous expression of some somatic genes around ZGA	Early stage NT embryos	[40]
Resistance to reprogramming of pluripotency genes	Early to blastocyst stage NT embryos	[41,42]
Defective epigenetic reprogramming of DNA and histones	NT embryos	[27,46,58]
Abnormal regulation of DNA methyltransferase expression	NT embryos	[59]
Incomplete erasure of the somatic type of DNA methylation and somatic cell-like features	NT embryos	[40,51,60]
Failure to reactivate X chromosome and aberrant X chromosome inactivation (XCI)	NT embryos	[61,62]
Aberrant remethylation leading to mis-expression of genes and retrotransposons important for ZGA	NT embryos	[63]
Disruption of imprinted gene methylation and expression	NT embryos	[64,65]
Loss of imprinting	NT embryos	[53]
Defective trophoblast cell lineage specification	NT blastocysts	[66,67]
Abnormal gene expression profiles in cloned placenta	Extra-embryonic tissues	[68,69,70]
Abnormal imprinted gene expression and methylation patterns in mid-gestation	Cloned fetuses and placentas	[57,71,72,73,74,75,76]