I don't don't know how much you understand this stuff. In 2003, estimates from gene-prediction programs suggested there might be 24,500 or fewer protein-coding genes (1). The Ensembl genome-annotation system estimates them at 23,299.
These protein-coding genes, also called exons, are subject to mutations including single nucleotide polymorphism(SNP), epigenetics, copy number variation, including inversion, deletion, duplication, triplication, etc. They are just learning a lot of this stuff. They used to call the DNA on chromosomes that is between exons, the protein-coding genes, "junk DNA" or introns. They have only relatively recently acknowledged that introns have regulatory functions and are highly conserved, i.e. rarely subject to mutation.
Discovery and Analysis of Evolutionarily Conserved Intronic Splicing Regulatory Elements