MicroRNA sponges are competitive inhibitors of small RNAs in mammalian cells. This study introduces a new method for microRNA inhibition using RNA molecules that contain multiple binding sites for a specific microRNA. These sponges are expressed as RNAs from transgenes and can be used to block microRNA activity. The sponges are designed to have strong promoters and multiple binding sites for the microRNA of interest. They are effective in derepressing microRNA targets and can be used to validate target predictions and assess microRNA loss-of-function phenotypes. The sponges are constructed by inserting multiple microRNA binding sites into the 3' UTR of a reporter gene. They are designed to have a bulge in the binding site to prevent endonucleolytic cleavage. The sponges can be expressed transiently from transfected plasmids or stably from chromosomal insertions. They are effective in inhibiting microRNA activity and can be used to study the function of microRNAs in mammalian cells. The study shows that microRNA sponges are at least as effective as chemically modified antisense oligonucleotides in inhibiting microRNA activity. They are specific to microRNA seed families and can be used to validate target predictions. The sponges are also effective in inhibiting microRNA levels and can be used to study the effects of microRNA loss of function in mammalian cells. The study also shows that microRNA sponges can be used to validate predicted microRNA targets. They are effective in derepressing microRNA targets and can be used to study the function of microRNAs in mammalian cells. The sponges are also effective in inhibiting microRNA levels and can be used to study the effects of microRNA loss of function in mammalian cells. The study concludes that microRNA sponges are a promising tool for studying microRNA function in mammalian cells. They are effective in inhibiting microRNA activity and can be used to validate target predictions and assess microRNA loss-of-function phenotypes. The sponges are also effective in inhibiting microRNA levels and can be used to study the effects of microRNA loss of function in mammalian cells.MicroRNA sponges are competitive inhibitors of small RNAs in mammalian cells. This study introduces a new method for microRNA inhibition using RNA molecules that contain multiple binding sites for a specific microRNA. These sponges are expressed as RNAs from transgenes and can be used to block microRNA activity. The sponges are designed to have strong promoters and multiple binding sites for the microRNA of interest. They are effective in derepressing microRNA targets and can be used to validate target predictions and assess microRNA loss-of-function phenotypes. The sponges are constructed by inserting multiple microRNA binding sites into the 3' UTR of a reporter gene. They are designed to have a bulge in the binding site to prevent endonucleolytic cleavage. The sponges can be expressed transiently from transfected plasmids or stably from chromosomal insertions. They are effective in inhibiting microRNA activity and can be used to study the function of microRNAs in mammalian cells. The study shows that microRNA sponges are at least as effective as chemically modified antisense oligonucleotides in inhibiting microRNA activity. They are specific to microRNA seed families and can be used to validate target predictions. The sponges are also effective in inhibiting microRNA levels and can be used to study the effects of microRNA loss of function in mammalian cells. The study also shows that microRNA sponges can be used to validate predicted microRNA targets. They are effective in derepressing microRNA targets and can be used to study the function of microRNAs in mammalian cells. The sponges are also effective in inhibiting microRNA levels and can be used to study the effects of microRNA loss of function in mammalian cells. The study concludes that microRNA sponges are a promising tool for studying microRNA function in mammalian cells. They are effective in inhibiting microRNA activity and can be used to validate target predictions and assess microRNA loss-of-function phenotypes. The sponges are also effective in inhibiting microRNA levels and can be used to study the effects of microRNA loss of function in mammalian cells.