Uniform boundedness and twisted sums of Banach spaces

We construct a Banach space X admitting an uncomplemented copy of l(1) so that X/l(1) = c(0). To do that we study the uniform boundedness principles that arise when one considers exact sequences of Banach spaces; as well as several elements of homological algebra applied to the construction of nontrivial twisted sum of Banach spaces. The combination of both elements allows one to determine the existence of nontrivial twisted sums for almost all combinations of classical Banach spaces.