In a groundbreaking study published on May 6, 2019, by Genome Biology, a novel bioinformatics algorithm known as SCRABBLE (Single-Cell RNA-seq Imputation Constrained by Bulk RNA-seq Data) has been introduced to significantly improve the analysis of single-cell RNA sequencing data. The study, led by Dr. Peng Tao and a team at the Children’s Hospital of Philadelphia and the University of Pennsylvania, marks a pivotal moment in the field of genomics, particularly in the area of single-cell analysis.

DOI: 10.1186/s13059-019-1681-8

The Challenge Addressed by SCRABBLE

Single-cell RNA sequencing (scRNA-seq) is a revolutionary technology that allows researchers to explore gene expression patterns in individual cells, providing a high-resolution window into complex biological processes. However, this technology is often hampered by the prevalence of ‘dropouts,’ where expressed genes are not detected, leading to large proportions of zero counts in the data. These dropouts present a considerable challenge for the accurate interpretation of gene expression.

The SCRABBLE Solution

SCRABBLE was designed to combat the challenges imposed by dropout events. By leveraging bulk RNA-seq data as a constraint, this algorithm can reduce bias towards expressed genes during the imputation process. Tao and colleagues demonstrated that SCRABBLE outperforms existing methods in several key areas: accurately recovering dropout events, capturing the true distribution of gene expression across cells, and preserving both gene-gene and cell-cell relationships within the data.

Using a combination of simulation and experimental data, the algorithm’s efficacy was rigorously tested. The findings indicated that SCRABBLE could become an invaluable tool for researchers working with single-cell transcriptomic data.

Methodology Behind SCRABBLE

SCRABBLE employs a matrix regularization technique for imputation, where the single-cell gene expression matrix is adjusted in such a way that the sum of imputed values matches the bulk RNA-seq expression profile. This method helps to reduce the imputation of false positives—genes that appear to be expressed in single cells when they are actually not—thereby enhancing the reliability of data interpretation.

Validation and Implications

The validation of SCRABBLE utilized various scRNA-seq datasets, covering a range of cell types and biological conditions. The researchers found that compared to alternative methods, SCRABBLE consistently provided more accurate imputations, suggesting that it could be a significant asset for researchers working on complex biological datasets.

By improving the accuracy of single-cell analyses, SCRABBLE could lead to better insights into developmental biology, cancer research, and personalized medicine. It can particularly impact our understanding of heterogeneous cell populations, such as tumors, where cell-to-cell variation plays a crucial role in disease progression and treatment response.

Accessibility and Tools

The authors have made SCRABBLE available to the scientific community, promoting transparency and collaboration. Researchers interested in utilizing SCRABBLE for their own work can access the code and documentation from the GitHub repository ( and Zenodo repository (DOI: 10.5281/zenodo.2585902).

Future Directions

While SCRABBLE represents a significant advance in scRNA-seq data analysis, the field continues to evolve rapidly. Future work could involve refining the algorithm to accommodate the growing complexity of datasets, integrating additional types of omics data, and improving the user experience for researchers with varying levels of computational expertise.


The development and validation of SCRABBLE have addressed a substantial challenge in the field of genomics, providing researchers with a powerful new tool for single-cell RNA-seq data analysis. As the technology behind single-cell analysis continues to evolve, tools such as SCRABBLE will be essential in advancing our understanding of the intricacies of gene expression in health and disease.


1. Peng T., Zhu Q., Yin P., Tan K. (2019) SCRABBLE: single-cell RNA-seq imputation constrained by bulk RNA-seq data. Genome Biol 20, 88. doi:10.1186/s13059-019-1681-8

2. Van den Berge, K., et al. (2018) Observation weights unlock bulk RNA-seq tools for zero inflation and single-cell applications. Genome Biol. 19, 24. doi: 10.1186/s13059-018-1406-4

3. Lun, A.T., et al. (2016) Pooling across cells to normalize single-cell RNA sequencing data with many zero counts. Genome Biol. 17, 75. doi: 10.1186/s13059-016-0947-7

4. Huang, M., et al. (2018) SAVER: gene expression recovery for single-cell RNA sequencing. Nat Methods. 15, 539-542. doi: 10.1038/s41592-018-0033-z

5. Gong, W., et al. (2018) DrImpute: imputing dropout events in single cell RNA sequencing data. BMC Bioinformatics. 19, 220. doi: 10.1186/s12859-018-2226-y


1. Single-cell RNA-seq analysis
2. RNA-seq data imputation
3. Gene expression bioinformatics
4. scRNA-seq dropout correction
5. Single-cell transcriptomics software