"If someone wants to be immortal in the current state of technology, first of all they need to avoid sudden death in any situation that isn't amenable to cryopreservation and subsequent revival, and second of all, when they get very old or develop other intractable health problems, they commission clones of themselves, who grow at an accelerated rate to adulthood and are then murdered and receive brain transplants. This is not very popular and it's only legal on one planet, but it happens on a routine if not overwhelmingly frequent basis. Mark is a clone, just not for that purpose."