A messenger RNA (mRNA) comprising (i) a 5' untranslated region comprising at least one RNA element that provides a translational regulatory activity; (ii) a full open reading frame comprising an initiation codon and encoding a polypeptide; and (iii) a 3' UTR, wherein the at least one RNA element is a GC-rich RNA element comprising guanine and cytosine nucleobases and, optionally, adenine and uracil nucleobases, or derivatives or analogs thereof, wherein the GC-rich RNA element is at least 50% or greater cytosine nucleobases and is at least 6 nucleotides in length, wherein the GC-rich RNA element is located about 5-10 nucleotides, about 5, about 4, about 3, about 2, or about 1 nucleotide upstream of a Kozak consensus sequence in the 5'UTR, or upstream of and immediately adjacent to the Kozak consensus sequence, and wherein the translational regulatory activity is selected from the group consisting of: (a) inhibits or reduces leaky scanning of the mRNA by the PIC or ribosome; (b) increases an amount of a polypeptide translated from the full open reading frame; (c) increases initiation of polypeptide synthesis at or from the initiation codon; (d) inhibits or reduces initiation of polypeptide synthesis at any codon within the mRNA other than the initiation codon; (e) inhibits or reduces an amount of polypeptide translated from any open reading frame within the mRNA other than the full open reading frame; (f) inhibits or reduces translation of truncated or aberrant translation products from the mRNA; and (g) a combination of any of (a)-(f). (i) a 5' untranslated region comprising at least one RNA element that provides a translational regulatory activity; (ii) a full open reading frame comprising an initiation codon and encoding a polypeptide; and (iii) a 3' UTR, wherein the at least one RNA element is a GC-rich RNA element comprising guanine and cytosine nucleobases and, optionally, adenine and uracil nucleobases, or derivatives or analogs thereof, wherein the GC-rich RNA element is at least 50% or greater cytosine nucleobases and is at least 6 nucleotides in length, wherein the GC-rich RNA element is located about 5-10 nucleotides, about 5, about 4, about 3, about 2, or about 1 nucleotide upstream of a Kozak consensus sequence in the 5'UTR, or upstream of and immediately adjacent to the Kozak consensus sequence, and wherein the translational regulatory activity is selected from the group consisting of: (a) inhibits or reduces leaky scanning of the mRNA by the PIC or ribosome; (b) increases an amount of a polypeptide translated from the full open reading frame; (c) increases initiation of polypeptide synthesis at or from the initiation codon; (d) inhibits or reduces initiation of polypeptide synthesis at any codon within the mRNA other than the initiation codon; (e) inhibits or reduces an amount of polypeptide translated from any open reading frame within the mRNA other than the full open reading frame; (f) inhibits or reduces translation of truncated or aberrant translation products from the mRNA; and (g) a combination of any of (a)-(f). (a) inhibits or reduces leaky scanning of the mRNA by the PIC or ribosome; (b) increases an amount of a polypeptide translated from the full open reading frame; (c) increases initiation of polypeptide synthesis at or from the initiation codon; (d) inhibits or reduces initiation of polypeptide synthesis at any codon within the mRNA other than the initiation codon; (e) inhibits or reduces an amount of polypeptide translated from any open reading frame within the mRNA other than the full open reading frame; (f) inhibits or reduces translation of truncated or aberrant translation products from the mRNA; and (g) a combination of any of (a)-(f).