The present invention generally relates to microRNAs such as vertebrate microRNA (miRNA), for example, mammalian miRNA. Various aspects of the invention are directed to the detection, production, or expression of miRNA. In one aspect, the invention provides systems and methods for identifying targets of miRNA sequences. For instance, in one embodiment, gene sequences comprising UTRs are compared with miRNA sequences to determine the degree of interaction, for example, by determining a free energy measurement between the miRNA sequence and the UTR, and/or by determining complementarity between at least a portion of the miRNA sequence and the UTR. In another aspect, the invention is directed to the regulation of gene expression using miRNA. For example, gene expression within a cell may be altered by exposing the cell to an oligonucleotide comprising a sequence that is substantially antisense to at least a portion of an miRNA region of the gene, for example, antisense to a 6-mer or 7-mer portion of the miRNA. In still another aspect, the invention is directed to the treatment of cancer. For instance, in one set of embodiments, an isolated oligonucleotide comprising a sequence that is substantially antisense to an miRNA, or a portion of an miRNA, is administered to a subject having or being at risk of cancer. Yet other aspects of the invention are directed to compositions or kits including oligonucleotides comprising a sequence that is substantially antisense to an miRNA (or a portion of an miRNA), methods of promoting any of the above aspects, or the like.