RecQ DNA helicases are genome surveillance proteins found in all kingdoms of life. They are characterized best in humans, as mutations in genes lead to developmental abnormalities and diseases. To better understand RecQ functions in plants we concentrated on and , the model species predominantly used for studies on DNA repair and gene targeting. Phylogenetic analysis of the six genes revealed their orthologs in humans and plants. Because Arabidopsis and differ in their and genes, reporter and deletion moss mutants were generated and gene functions studied in reciprocal cross-species and cross-kingdom approaches. Both proteins can be found in meristematic moss tissues, although at low levels and with distinct expression patterns.RecQ4 is involved in embryogenesis and in subsequent development as demonstrated by sterility of Δ mutants and by morphological aberrations. Additionally, Δ displays an increased sensitivity to DNA damages and an increased rate of gene targeting. Therefore, we conclude that RecQ4 acts as a repressor of recombination. In contrast,RecQ6 is not obviously important for moss development or DNA repair but does function as a potent enhancer of gene targeting.