On countable cofinality of definable chains in Borel partial orders

We prove that in some cases definable chains of Borel partial orderings are necessarily countably cofinal. This includes the following cases: analytic chains, ROD chains in the Solovay model, and $\Sigma^1_2$ chains in the assumption that $\omega_1^{L[x]}<\omega_1$ for all reals $x$.