Metric Markov Cotype Two of ℓ${}_1$

We prove that the real Banach space ℓ${}_1$ has metric Markov cotype two, answering a question of Mendel and Naor. As a consequence, every Lipschitz map from an arbitrary subset of a real Hilbert space into ℓ${}_1$ extends to the whole Hilbert space with a universal multiplicative loss in its Lipschitz constant, resolving Ball's extension problem for this target.