Dr. Welsh received his Ph.D. in chemistry from Columbia University. He did a brief post-doctoral fellowship at UC San Francisco funded by the California American Cancer Society, and then returned to Columbia to work with Argiris Efstratiadis, where he created the first cDNA library for the mouse egg. He then went on to the California Institute of Biological Research with Dr. Michael McClelland, where he was promoted to Assistant Professor. He joined the Sidney Kimmel Cancer Center in 1998 as Associate Professor, where has helped establish the Genomics and Bioinformatics Cores. He has taken a non-traditional approach to cancer research, merging elements of molecular biology, mechanical engineering and computing. Dr. Welsh became part of the Vaccine Research Institute of San Diego in 2009.

Our laboratory works on novel molecular methods to study cancer. Among these new methods has been Arbitrarily Primed Polymerase Chain Reaction (AP-PCR), a method that has been cited thousands of times primarily in the area of organism taxonomy, and was used to discover the Mutator Phenotype in cancer. A variation of this method, RNA Arbitrarily Primed Polymerase Chain Reaction (RAP-PCR) is similar to Differential Display and was used often to discover differential regulation of genes prior to the microarray revolution. Our own twist on microarrays was to use RAP-PCR as a means of significantly enhancing microarray sensitivity. In a second twist called Vertical Arrays, we use RAP-PCR to prepare the "spots" on microarrays and then probe the arrays to discover gene expression behavior one gene at a time, but in thousands of biological samples, simultaneously. A "boutique" method we worked out allows us to analyze the translation of individual genes in hundreds of samples simultaneously.