 A 2003 study proposed that a gene-environment interaction (GxE) may explain why life stress is a predictor for depressive episodes in some individuals, but not in others, depending on an allelic variation of the serotonin-transporter-linked promoter region (5-HTTLPR).As of 2019, six meta analyses of the 5-HTTLPR GxE interaction have been performed.Two 2009 meta analyses reported null findings, while a 2011 meta analysis with more liberal inclusion criteria reported a significant relationship.