Atlantic cod is one of the most economically important fish species worldwide. Nevertheless, the profitability of the cod farming industry is severely restricted by precocious sexual maturation of the fish in captivity, which reach puberty prior to attaining commercial size. Photoperiod manipulation, typified by continuous illumination, has been successfully used to delay sexual maturation to some extent in Atlantic cod, similarly to what has been observed in other farmed fishes, including Atlantic salmon and European sea bass. While most studies involving photoperiod manipulation in Atlantic cod have been conducted on two-year old fish, it has been reported that short-term photoperiod manipulation during early juvenile stages has a significant positive effect on somatic growth, which is dependent on genetic background and environmental temperature. Remarkably, juvenile cod kept under continuous light for three months had a significantly higher weight than the simulated natural photoperiod group and this difference remained even after 30 months of sea-pen rearing under identical ambient conditions until harvest. By this point, fish subjected to the initial continuous light treatment were up to 9% larger than their counterparts reared under natural photoperiod. The present study was designed to further our limited understanding about the epigenetic regulation of somatic growth in Atlantic cod, with particular focus on themll family, since these genes are known to play a crucial role. We have cloned all representatives of the mll gene family in Atlantic cod and examined their expression levels in fast muscle of juvenile fish kept under continuous illumination or simulated natural photoperiod. It is not entirely clear how light stimulates somatic growth but it is not due to a simple extension of foraging activity and corresponding feed intake. In fact, in their natural environment age 1 Atlantic cod, like the ones used in our study, preyed preferentially on benthos at night time. Moreover, our experimental fish were fed equal amounts daily and there were no apparent differences in feed consumption between the two light groups. It is likely that the short-term photoperiod treatment induces metabolic changes that promote higher growth rates, probably due to more efficient nutrient utilisation. Day LEE011 length in Norway, had reached 24 hours by 120 days into the experiment and, therefore, all fish were kept under identical conditions from this point onwards. Nevertheless, there was still an average 10.5% weight difference between the natural photoperiod and continuous illumination groups at 180 days. This indicates that short-term light manipulation may have a persistent effect on muscle growth and corroborates a previous report, which showed that juvenile cod reared under continuous light for three months and then transferred to sea pens became up to 9% larger than their counterparts initially kept under simulated natural photoperiod conditions. It has been used in aquaculture to control growth and maturation of several commercial fish species. However, the molecular mechanisms underlying growth plasticity induced by light are still unknown. Four basic helix-loophelix transcription factors known as MRFs have received considerable attention as key players involved in determination and differentiation of skeletal muscle.