Not quite true. It is possible to change the expression of genes without changing the underlying sequences -- see this Wikipedia article on
twins
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Identical twins have identical DNA but differing environmental influences throughout their lives affect which genes are switched on or off. This is called epigenetic modification. A study of 80 pairs of human twins ranging in age from 3 to 74 showed that the youngest twins have relatively few epigenetic differences. The number of differences between identical twins increases with age. 50-year-old twins had over three times the epigenetic difference of 3-year-old twins. Twins who had spent their lives apart (such as those adopted by two different sets of parents at birth) had the greatest difference. (Fraga, et al., 2005).
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DNA methylation, histone modification and other processes related to gene activation occur throughout an individual's lifetime. The study of these changes is called
epigenetics.