Which papers made the biggest splash this year? ScienceWatch, a website that tracks and analyzes trends in basic science research, compiles bimonthly lists of the 10 most cited papers. From those lists, The Scientist pulled the five papers in biology published in the last two years which were some of the most cited papers in 2009. The two topics that dominate the top five papers this year: genomics and stem cells.
(*All citation data, both ours and that of ScienceWatch, are from ISI.)

Stack of journals Image: flicker/meviola

5. A M. Wernig, et al., "In vitro reprogramming of fibroblasts into a pluripotent ES-cell-like state," Nature 448: 318-24, 2007.
Citations this year: 237
Total citations to date: 512
Findings: Scientists successfully performed somatic-cell nuclear transfer (SCNT), producing stem cell lines and cloned animals for the first time using fertilized mouse eggs. This paper consistently ranked in the top 10 most cited papers in 2009, according to ScienceWatch.

4. E. Birney, et al., "Identification and analysis of functional elements in 1% of the human genome by the ENCODE pilot project, "Nature, 447: 799-816, 2007.
Citations this year: 267
Total citations to date: 618
Findings: The ENCODE project -- ENCODE stands for the ENCyclopedia Of DNA Elements -- set out to identify all functional elements in the human genome. After examining one percent of the genome, the paper revealed several new insights about how information encoded in the DNA comes to life in a cell.

3. A. Barski, et al., "High-resolution profiling of histone methylations in the human genome," Cell, 129: 823-37, 2007.
Citations this year: 299
Total citations to date:: 560
Findings: This study looked at how histone modifications influence gene expression in more detail than previous attempts. Using a powerful sequencing tool called Solexa 1G, the researchers mapped more than 20 million DNA sequences associated with specific forms of histones, finding there were differences in methylation patterns between stem cells and differentiated T cells.

2. K.A. Frazer, et al., "A second generation human haplotype map of over 3.1 million SNPs," Nature, 449: 854-61, 2007.
Citations this year: 389
Total citations to date: 588
Findings: Since the sequencing of the human genome in 2003, the International HapMap Project has explored single nucleotide polymorphisms (SNPs) -- differences in a single letter of the DNA -- to study how these small variations affect the development of diseases and the body's response to pathogens and drugs. HapMap I, the original report, placed one SNP at roughly every 5,000 DNA letters. The newest map, featured in this paper, sequenced an additional 2 million SNPs, increasing the map's resolution to one SNP per kilobase. The additional detail allows scientists to more closely investigate patterns in SNP differences, especially in hotspot regions, or concentrated stretches of DNA.

1. K. Takahashi, et al., "Induction of pluripotent stem cells from adult human fibroblasts by defined factors," Cell, 131: 861-72, 2007.
Citations this year: 520
Total citations to date: 886
Findings: This work from Shinya Yamanaka's lab in Japan was the first to demonstrate that induced pluripotent stem (iPS) cells can be generated from adult human dermal fibroblasts. Previous efforts by the team showed that iPS cells could be derived from mouse somatic cells. This paper was an easy top pick, receiving the most citations this year, according to ScienceWatch.