Are russians descendants of byzantines?

The Russians are a very mixed race. The word Rus however points to Nordic tribes who descended into the country around the 5 or 6th century and they are supposed to be the founders of the country as such. Of course the title Tzar adopted first by Ivan the Terrible is a direct derivative of the word Caesar (Kaiser) and confirms his desire to build a new Roman Empire.Russian Tzars are partly descended during the first years from the Byzantines through their marriages with Byzantine Princesses.