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The ryr1b mutants are called relatively relaxed mutants. The mutant phenotype results from a splicing mutation in the zebrafish ryr1b gene that encodes the Ca-releasing channels in the sarcoplasmic reticulum. The mutants display normal behavior at 1 dpf but show slow swimming at the onset of 2 dpf due to weak contractions of fast-twitch skeletal muscles. The zebrafish ryr1b gene is expressed in fast-twitch skeletal muscles, whereas ryr1a in slow-twitch skeletal muscles. Thus, ryr1b mutants show defective contraction of fast-twitch muscles but not of slow-twitch muscles.
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