Background: Single-cell RNA-sequencing (scRNA-seq) is a rapidly evolving technology that enables measurement of gene expression levels at an unprecedented resolution.Despite the explosive growth in the number of cells that can be assayed by a single experiment,scRNA-seq still has several limitations,including high rates of dropouts,which result in a large number of genes having zero read count in the scRNA-seq data,and complicate downstream analyses.Methods: To overcome this problem,we treat zeros as missing values and develop nonparametric deep leaing methods for imputation.Specifically,our LATE (Leaing with AuToEncoder) method trains an autoencoder with random initial values of the parameters,whereas our TRANSLATE (TRANSfer leaing with LATE) method further allows for the use of a reference gene expression data set to provide LATE with an initial set of parameter estimates.Results: On both simulated and real data,LATE and TRANSLATE outperform existing scRNA-seq imputation methods,achieving lower mean squared error in most cases,recovering nonlinear gene-gene relationships,and better separating cell types.They are also highly scalable and can efficiently process over 1 million cells in just a few hours on a GPU.Conclusions: We demonstrate that our nonparametric approach to imputation based on autoencoders is powerful and highly efficient.