Abnormal X‐Chromosome Dosage Compensation as a Possible Cause of Early Developmental Failure in Mice

An extra maternally derived X chromosome (X M ) but not a paternally derived one (X P ) is detrimental in early mouse embryogenesis resulting in failure to form the ectoplacental cone and extra‐embryonic ectoderm. Cytogenetic studies suggested that two X M chromosomes remain active in the trophectoderm and possibly also the primitive endoderm, in which X P is preferentially inactivated in normal female embyos. Two copies of an active X chromosome due to maternal imprinting seem to prevent further differentiation of the trophectoderm.