text: While it is well known that the genome of one individual can have extensive differences when compared to the genome of another, recent research has found that some coding regions are highly constrained, or resistant to mutation, between individuals of the same species.  This is similar to the concept of interspecies constraint in conserved sequences.  Researchers termed these highly constrained sequences constrained coding regions (CCRs), and have also discovered that such regions may be involved in high purifying selection. On average, there is approximately 1 protein-altering mutation every 7 coding bases, but some CCRs can have over 100 bases in sequence with no observed protein-altering mutations, some without even synonymous mutations. These patterns of constraint between genomes may provide clues to the sources of rare developmental diseases or potentially even embryonic lethality. Clinically validated variants and de novo mutations in CCRs have been previously linked to disorders such as infantile epileptic encephalopathy, developmental delay and severe heart disease.