Time- and sex-dependent loss of DNA methylation in absence of DNMT3L (DNA methyltransferase 3-like). All embryonic stem (ES) cell clones analyzed had typical ES cell morphology and comparable growth rates (data not shown). (a) Culture of Dnmt3L-/- ES cells caused a loss of DNA methylation from multiple repeat-sequence classes. LINE1 promoters, Intracisternal A Particle (IAP) long terminal repeats (LTRs) and minor satellite are shown. (b) Loss of methylation in XX Dnmt3L-/- ES cells was more rapid than in XY Dnmt3L-/- ES cells. Late passage XX Dnmt3L-/- ES cells showed an extent of demethylation that was similar to that of ES cells null for Dnmt1. Dnmt3L genotype is indicated by '+/+' and '-/-' at the top of each lane. (c) Comparison of DNA methylation in XY and XX ES cells. Three normal XY ES cell lines were compared with eight normal XX ES cell lines. Demethylation was more pronounced in all of the XX lines compared with XY cells.