Board Thread:Naruto Discussions/@comment-24757127-20140709042852/@comment-2120712-20140709185311

My guess is that the Senju, without a definite known leader after Tobirama's death in the first war, probably married into the other clans, or took up other names and became the various clans we know today. They were known as "the clan with a thousand skills" wouldn't be hard for certain factions to band together under one of many ninjutsu styles the clan has mastered and form a clan.