World leaders marked Holocaust Memorial Day with a ceremony at Auschwitz, where at least 1.1 million people, mostly Jews, died. However often overshadowed is the mass killing of Europe's Roma and Sinti populations. In Nazi-occupied Bohemia and Moravia, for example, as many as 90% of the pre-war population of around 6,000 were killed. Rob Cameron has this report from the Czech capital, Prague.