SiS-Builder is a web based tool, developed in the framework of the DICTA-SIGN project in order to cover for the need of creating sign language (SL) lexical resources, adequate for sign synthesis performed by a signing avatar (virtual signer). The tool’s initial function was to automatically generate SiGML transcriptions of HamNoSys strings, as well as the relevant transcription files, by providing the HamNoSys characters of a sign. SiS-Builder provides an environment, which is accessible by everyone and allows interaction without requiring any special installations on the client side. The tool enables users to create or review HamNoSys notations and SiGML scripts of sign lemmas on line, switch between SiGML data and HamNoSys notations by selecting the wished function, review already created lemmas and proofread avatar performance, also viewing the video of the performed sign. It makes then possible to experiment with the results of synthesis of lexicon items, by either consulting the HamNoSys sequence, for those familiar with the HamNoSys syntax, or animating the results through the avatar with the use of the SiGML script.