The titles of the first four episodes of Castle season 8 have been revealed.

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As production continues on the eighth season of ABC's Castle, episode titles are beginning to leak out, giving fans a tease of what's ahead for Castle (Nathan Fillion), Beckett (Stana Katic) and the gang.

Some fans many know that the premiere will be a two-parter, while still airing on separate nights. The premiere will be titled XY. It is not known what this could be referring to specifically, but "XY" is the sex chromosome for males.

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The second episode (and final part of the premiere) is titled XX, which is commonly known as the sex chromosome for females. Should fans expect some sort of gender dynamic going on in the two-part premiere?

Two more titles have been revealed recently. The third episode title is What Lies Beneath, according to Castle executive producer (and episode writer) Barry O'Brian on Twitter. This title could mean pretty much anything.

Chad Gomez Creasey, another writer on the show, tweeted the episode 4 title, which is PhDead. Creasey wrote this episode. Cast and crew are starting production on episode 4 today, (Thursday, Aug. 20). This title is a probable reference to the case of the week, which will probably center around the death of a PhD recipient.

Castle season 8 premieres Monday, Sept. 21 at 10 p.m. ET/PT on ABC.

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Castle, Television, ABC