The sarcoplasmic reticulum (SR) plays an integral role in muscle contraction. The SR is a specialized endoplasmic reticulum found in muscle cells that stores calcium?a major chemical signal for muscle contraction and relaxation. The calcium release channels on the SR are called ryanodine receptors (RyR). The RyR regulates the concentration of calcium in the cytoplasm of the muscle cell. Three major isoforms of RyR exist - RyR1, RyR2, and RyR3. RyR1 is expressed primarily in skeletal muscle, while RyR2 is expressed in cardiac muscle, whereas RyR3 is expressed in a wide variety of tissues (Takeshima et al., 1989). Recently, two RyR1-like isoforms have been discovered called RyR1-fast and RyR1-slow, which are discretely expressed in fast and slow muscle fibers of fish respectively. Gene duplication is believed to be the cause of the evolution of the different isoforms of the RyR1 gene. One major goal of our study is to determine if this duplication is specific to the teleost lineage or if it is also found in the non-teleost Actinopterygiian fish. To address this question we used the polymerase chain reaction (PCR) to amplify portions of the RyR message from first strand cDNA from horned shark. Preliminary evidence suggests that the horned shark does not have two RyR1 isoforms and may only express the RyR3 isoform. It is known that amphibians and mammals (tetrapods) do not possess the RyR1-fast and RyR1-slow isoforms. Therefore, gene loss occurred sometime after the appearance of Teleostei and before the appearance of the tetrapods.