A problem with a gene that controls a blood protein called C1 inhibitor often causes HAE. In most cases, you don’t have enough of this protein. In others, you have normal levels but it doesn't work right.
For the most common form of HAE, if one of your parents has HAE, you have a 50% change of having it, too. But sometimes the gene change happens for unknown reasons. If you have the broken gene, you can pass it on to your children.