The present invention provides splice variants of myostatin that promote muscle growth, and include polynucleotides and polypeptide sequences, constructs comprising the sequences and compositions for regulating muscle growth and treating diseases associated with muscle tissue. The splice variants include the consensus sequence X1 I F L E X2 X3 X4 Q X5 C S I L X6 X7 X8 X9 X10 wherein X1 is I or L, X2 is V or L, X3 is Y, C, G or S, X4 is I or F, X5 is F or L, X6 is G or E, X7 is E or V, X8 is A or T, X9 is A or V and X10 is absent, F or L. The present invention also provides for the use of the present sequences in identifying animals with altered muscle mass, and for use in selective breeding programs to produce animals with altered muscle mass.