Human Son of sevenless homolog 1 (SOS1), also known as guanine nucleotide exchange factor 1 (GF1), GenBank Accession No. NM_005633, a.a. 594-1049, with N-terminal FLAG-tag and C-terminal Avi-tag, expressed in a Baculovirus infected Sf9 cell expression system, MW=57 kDa. This protein is enzymatically biotinylated using Avi-Tag™ technology. Biotinylation is confirmed to be ≥90%.