no offence,., but... do you even read the manga.. did you just join and read the last 5 chapters... luckily i know most of the chapters by heart and i can show you a link to anything i say or supportJudging by who has been resurrected and who, as stated, can't be (i.e. Jiraiya), it would seem the person must be dead before the DNA can be collected. Otherwise his could have been collected by Kabuto in an earlier battle. This would mean that Kabuto would have had to visit all the graves. As far as Nagato and Itachi, it may be to recent for Kabuto to have gotten their samples as Madara had been paying closer attention to him when those battles/deaths occurred.
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now im not tryna make fun or flame you, but can you start reading from..umm.. ch 490 till now, that will help out alot.. (dont take it personaly)