def _valid_source_node(node: str) -> bool:
    """Check if it is valid source node in BEL.

    :param node: string representing the node
    :return: boolean checking whether the node is a valid target in BEL
    """
    # Check that it is an abundance
    if not node.startswith('a'):
        return False
    # check that the namespace is CHEBI and PUBMED
    if 'CHEBI' in node or 'PUBCHEM' in node:
        return True

    return False