Small interfering RNAs (siRNAs) and microRNAs (miRNAs) are functional RNA molecules that have recently emerged as important regulators of gene expression at the posttranscriptional or translational level. The RNA interference effects of siRNA on gene expression make it a valuable research tool for knocking down the expression of genes in mammalian cells in vitro and in vivo enabling the elucidation of molecular mechanisms underlying human diseases. Endogenous miRNAs are involved in a variety of physiological and pathological processes in humans. In this mini-review we first address the synthesis, mechanisms of action, and functions of siRNAs. Then, we focus on recent advances and technologies in miRNA and protein research of the human placenta. Next, we discuss the clinical applications of miRNA in lung cancer. We also touch on "long" noncoding RNAs from intergenic regions of the human genome. This review article is based on a presentation given at a symposium entitled Basic and Clinical Studies on Functional RNA Molecules for Advanced Medical Technologies held at Nippon Medical School in Tokyo, Japan, on November 7, 2009.