Birth order has strong correlations with developmental and environmental aspects. Birth order is a presumed factor affecting multiple diseases including leukemia (1, 2, 3), atopy (4), type I diabetes (5), non-Hodgkin lymphoma (6) and synovial sarcoma (7). Moreover, first born subjects have unique in utero characteristics including less sufficient placentation, higher estrogen levels, and lower insulin sensitivity. Birth order might therefore play a role in shaping DNA methylation pattern and influence the initiation of the diseases mentioned above.

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