Modification formats having modified nucleotides are provided for siRNA. Short interfering RNA having modification formats and modified nucleotides provided herein reduce off-target effects in RNA interference of endogenous genes. Further modification formatted siRNAs are demonstrated to be stabilized to nuclease-rich environments. Unexpectedly, increasing or maintaining strand bias, while necessary to maintain potency for endogenous RNA interference, is not sufficient for reducing off-target effects in cell biology assays.