The present disclosure relates to polynucleotides comprising an open reading frame of linked nucleosides encoding human interleukin-12 (IL12), functional fragments thereof, and fusion proteins comprising IL12. In some embodiments, the open reading frame is sequence-optimized. In particular embodiments, the disclosure provides sequence-optimized polynucleotides comprising nucleotides encoding the polypeptide sequence of human IL12, or sequences having high sequence identity with those sequence optimized polynucleotides.