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It was used as a code in ABC's Defying Gravity. The acronym was used in Season 1 Episode 4 between Maddux (Ron Livingston) and Ajay( Zahf Paroo) to try and solve a problem with the space ship. It also helped build some more of the history between the two characters as we contently see flash backs to training before the mission to Mars. It might also be used as a help tact when you don't know the facts about something and you know your friend does.
Maddux: "Do you have any idea what's wrong?"
Ajay: "It looks like a H21K sequence to me."

You: "The reason why the sky is blue is because of the H21K Sequence."
Friend: "By that you mean the molecules in the air scatter blue light from the sun."
You: "Correct."
by SpindlyMan August 17, 2009
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