PIK3CD and Merkel cell skin cancer: To quantify the mRNA expression of class I PI3K catalytic subunit isoforms (PI3K-α, PI3K-β, PI3K-δ, and PI3K-γ) in MCC cell lines, real time quantitative RT-PCR (qPCR) was conducted using cDNAs isolated from three primary MCC cell lines (MCC-3, MCC-9, and MCC-21) established in our laboratory as well as MKL-1, a commercially available classic MCC cell line.