The present invention provides products and methods for modulating expression of a target gene in a cell. One such method includes introducing into the cell a polynucleotide that forms a duplex region with an mRNA transcribed from said target gene, where the duplex region comprises a mammalian miRNA target region. Another such method includes introducing into the cell an siRNA that forms a duplex region with an miRNA, or precursor thereof, where an mRNA transcribed from the target gene comprises a miRNA target region. In certain preferred embodiments, the methods further include measuring expression of the target gene. The methods are particularly useful for modulating ontogenesis, function, differentiation and/or viability of a mammalian cell. As such, the invention also provides methods for controlling ontogenesis of mammal, function of mammalian cell, differentiation of mammalian cell or viability of mammalian cell in the post-transcriptional phase by introducing into the cell a miRNA or a siRNA silencing precursor to the miRNA. The invention additionally provides polynucleotides, including vectors, useful in the method of the instant invention. The provided polynucleotides include a plasmid vector comprising a promoter and a polynucleotide sequence expressing miRNA or precursor to the miRNA. Also included is a plasmid vector comprising a promoter and a nucleotide sequence expressing siRNA silencing precursor to miRNA. In certain preferred embodiments, the mRNA is capable of forming a duplex region with an mRNA transcribed from a mammalian target gene.