Genetics, the cause of CdLs

In 2004, the news that “the” Cornelia de Lange Syndrome (CdLS) gene had been discovered was very much welcomed, following years of searching and many families donating blood samples towards this cause.
Now, eight years later, the picture seems much more confusing. Despite the knowledge of the most recently reported fourth CdLS gene (HDAC8) and another closely related gene (RAD21), the percentage of people with CdLS having detectable changes (mutations) on testing remains under 70 percent, and the diagnosis is still made clinically. You may ask, “How can this be?”