Factors that can alter the level of expression of a gene include

Gene expression is the process by which information within a gene is made into functional product, such as RNA through transcription or protein through translation. There are various factors that may alter the level of expression of a gene which includes;environmental factors such as  an increase in environmental temperature, an increase in salinity. other factors such as exposure to UV light/radiation and also presence of a particular energy substrate. 

These could be environmental factors such as;

Exposure to UV radiation that causes mutations

an increase in salinity.

an increase in environmental temperature.

presence of a particular energy substrate.

Gene expression can be affected in different stages from transcription, RNA processing, to post-translation modification.